Fuel Cell System and Solid Polymer Electrolyte Film

ABSTRACT

According to the invention, a fuel cell system features a fuel cell ( 14 ) having a solid polymer electrolyte membrane ( 4 ), and an antioxidant residing in or contacting the solid polymer electrolyte membrane ( 4 ), for inactivating active oxygen.

TECHNICAL FIELD

The present invention relates to a fuel cell system and a solid polymerelectrolyte film, and more specifically, to a fuel cell system, a solidpolymer electrolyte membrane, a fuel cell, and a fuel cell vehicle.

BACKGROUND ART

The fuel cell technology is attracting attention as a solution to theproblem of energy resources, as well as to the issue of global warm dueto CO₂ emission. The fuel cell is adapted for electrochemical oxidationof a fuel, such as hydrogen or methanol or any hydrocarbon else in thecell, to effect a direct conversion of chemical energy of the fuel toelectrical energy to be taken out. The fuel cell is thus free fromemissions of combustion products of fuel, such as NO_(X) and SO_(X), andattracts attention as a clean energy source for internal combustionengines such as for automobiles, or for thermal power plants.

There are some types of fuel cells, with the PEFC (proton-exchangemembrane fuel cell) inclusive, which is now most watched, and developed.The PEFC has various advantages, such that it is (1) adapted for anoperation to be facile in start and stop at low temperatures, (2)allowed to be high in theoretical voltage as well as in theoreticalefficiency of conversion, (3) implemented with a liquid-free electrolyteallowing a flexible design of cell structure, such as a vertical type,and (4) configured for an interface between ion exchange membrane andelectrode to have a three-phase interface controlled to take out anenhanced amount of current, achieving a high density power output.

The principle of operation of a fuel cell includes two electrochemicalprocesses, being an H₂ oxidation at the fuel electrode (cathode asnegative-pole), and a four-electron reduction of molecular oxygen (O₂)shown by formula (A1) below, which produces water.O₂+4H⁺+4e ⁻→2H₂O  (A1)

Actually, concurrent side reactions occur. Typically, a two-electronreduction of O₂ takes place at the air electrode, producing hydrogenperoxide (H₂O₂), as shown by formula (A2) below.O₂+2H⁺+2e ⁻→H₂O₂  (A2)

Hydrogen peroxide is stable, and has a long life, though weak inoxidizability. Hydrogen peroxide decomposes, following reaction formulas(A3) and (A4) shown below. When decomposing, it generates radicals, suchas hydroxy radical (.OH) and hydroperoxy radical (.OOH). Such radicals,in particular hydroxy radical, are strong in oxidizability, so that evenperfluorosulfonated polymer used as an electrolyte membrane may bedecomposed in a long use.H₂O₂→2.OH  (A3)H₂O₂→.H+.OOH  (A4)

Low-valence ions of transition metal such as Fe²⁺, Ti³⁺, or Cu⁺, ifpresent any, cause a Haber-Weiss reaction, where hydrogen peroxide isone-electron reduced by such a metal ion, generating hydroxy radical.Hydroxy radical, most reactive among free radicals, has a very strongoxidizability, as is known If the metal ion is an iron ion, theHaber-Weiss reaction is known as a Fenton reaction shown by formula (A5)below.Fe2⁺+H₂O₂→Fe³⁺+OH—+.OH  (A5)

Metal ions, if mixed in an electrolyte membrane, cause a Haber-Weissreaction, whereby hydrogen peroxide in the electrolyte membrane ischanged into hydroxy radical, whereby the electrolyte membrane may bedeteriorated (Kyoto University Graduate School of Engineering as en fromthe New Energy and Industrial Technology Development Organization, “2001yearly results report, researches and developments of proton-exchangemembrane fuel cell, researches on deterioration factors ofproton-exchange membrane fuel cell, fund research (1) on deteriorationfactors, deterioration factor of electrode catalyst/electrolyteinterfaces”, March 2002, p. 13, 24, 27).

As a method for blocking hydrogen peroxide from becoming radical, therehas been proposed a method of having a metal oxide, such as a manganeseoxide or cobalt oxide, mixed and dispersed in the electrolyte membrane,for decomposing hydrogen peroxide by contact therewith, or a method ofhaving a peroxide stabilizing agent, such as a tin compound, mixed anddispersed in the electrolyte membrane, for impeding a change of hydrogenperoxide into radical (Japanese Patent Application Laying-penPublication No. 2001-118591). For example, there has been proposed amethod in which a compound with phenolic hydroxyl is mixed in theelectrolyte membrane, so that peroxide radicals are trapped to beinactive (Japanese Patent Application Laying-Open Publication No.2000-223135). Another method is proposed in which an electrolytemembrane has a phenol compound, amine compound, surfer compound,phosphorus compound, or the like mixed therein as an antioxidant tovanish generated radicals (Japanese Patent Application Laying-OpenPublication No. 2004-134269). Another proposed method has an electrolytemembrane disposed adjacent to a catalyst layer containing moleculeshaving a smaller bond energy than carbon-fluorine bonding in theelectrolyte membrane, the molecules reacting with priority to hydroxyradicals, thereby protecting the electrolyte membrane (Japanese PatentApplication Laying Open Publication No. 2003-109623).

DISCLOSURE OF INVENTION

However, metal oxides and tin compounds are insoluble in water, and itwill be difficult to achieve a uniform dispersion in an electrolytemembrane even after their atomization into fine particles. Further, tincompounds may be decomposed, with elution of metal as a cation in theelectrolyte membrane to be acidic, which may cause a Haber-Weissreaction, promoting generation of active oxygen.

When decomposing hydrogen peroxide by a catalyst such as a metal oxide,the decomposition is caused as a contact decomposition. That is, forhydrogen peroxide, it will not occur unless in the vicinity of a contactTherefore, decomposition is disabled of hydrogen peroxide remote fromcatalyst.

For a trapping inactivation of peroxide radical, there is needed anamount of radical-inactivative compound equivalent to a molarconcentration of generated radical. This compound is not regenerative.The radical-inactivative compound is thus needed every time whentrapping radical, constituting a difficulty to consume peroxides over along term.

Generation of hydroxy radical occurs with an increased tendency in avicinity of a three-phased interface of an air electrode, that is anenvironment where oxygen and platinum as an electrode catalyst exist andcompounds tend to be oxidized, so that those methods in which anelectrolyte membrane simply contains an oxidation-preventive compound,as described above, may have this compound also oxidized to be vanished,whether hydroxy radical is present or not, thus resulting in aninefficiency. Still less, that compound may react with hydroxy radicalto generate an unstable radical or peroxide, which may act as aninitiator of additional reaction for oxidation, causing deterioration ofthe electrolyte membrane.

This invention is made in view of such points. It is an object of theinvention to provide a fuel cell system and a solid polymer electrolytemembrane with an excellent tolerance.

A first invention is a fuel cell system characterized by a fuel cellhaving a solid polymer electrolyte membrane, and an antioxidant residingin or contacting the solid polymer electrolyte membrane, forinactivating active oxygen.

A second invention is a solid polymer electrolyte characterized by acompound having a redox cycle, where it acts as a reducing agent in arange of potentials lower than a redox potential of hydroxy radical andas an oxidizing agent in a range of potentials higher than a redoxpotential where hydrogen peroxide acts as a reducing agent.

As a third invention, a fuel cell comprises a solid polymer electrolyteaccording to the second invention.

As a fourth invention, a fuel cell vehicle has mounted thereon a fuelcell system according to the first invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram generally illustrating a mode of embodiment of afuel cell system according to the invention.

FIG. 2 is an exploded perspective view of a unit cell of a fuel cellconstituting the fuel cell system according to the invention.

FIG. 3 is a diagram illustrating transfer of materials in a membrane-erode assembly constituting the unit cell.

FIG. 4 is a schematic diagram illustrating a three-phased interface inan air electrode.

FIG. 5 is a diagram illustrating a mechanism for inactivation of activeoxygen by NHPI.

FIG. 6 is a diagram illustrating a mechanism for inactivation of activeoxygen by NHGI.

FIG. 7 is a diagram of formulas of exemplary compounds.

FIG. 8 is a diagram of formulas of exemplary compounds.

FIG. 9 is a diagram of formulas of exemplary compounds.

FIG. 10 is a diagram of formulas of exemplary compounds.

FIG. 11 is a diagram illustrating a mechanism for inactivation of activeoxygen by TEMPO.

FIG. 12 is a diagram showing redox potentials of hydroxy radical,oxygen, hydrogen peroxide, hydrogen, NHPI, and PINO.

FIG. 13 is a diagram illustrating a hydroxy radical reduction mechanismby Grotthuss mechanism.

FIG. 14 is a cyclic voltammogram in electrode reactions of NHPI.

FIG. 15 is a cyclic voltammogram in electrode reactions of TEMPO.

FIG. 16 is a graph showing, for unit cells of a fuel cell fabricated inan embodiment example 1, a potential vs current curve by initial valuesof a start-stop repeating endurance test, and a potential vs. currentcurve after endurance.

FIG. 17 is a graph showing results of a stability test for NHPI andNHGI.

BEST MODE FOR CARRYING OUT THE INVENTION

There will be described below details of a fuel cell system and a solidpolymer electrolyte according to the invention, in accordance with theirmodes of embodiment.

(Fuel Cell System)

According to a mode of embodiment of the invention, the fuel cell systemis characterized by a fuel cell having an electrode, and an antioxidantresiding in or contacting the electrode, for inactivating active oxygen.The fuel cell system taken now as an example of the invention is a solidpolymer electrolyte type that employs, as its electrolyte membrane, asolid polymer electrolyte membrane. FIG. 1 is a diagram generallyillustrating a mode of embodiment of the fuel cell system according tothe invention. According to this mode of embodiment, the fuel cellsystem is generally made, as illustrated in FIG. 1, by a fuel cell 1,and an antioxidant supply system 11 disposed outside the fuel cell 1 andconfigured to supply the fuel cell 1 with an antioxidant to be broughtinto contact with electrodes in the fuel cell 1, for inactivation ofactive oxygen therein.

As illustrated in FIG. 1, the fuel cell 1 constituting the fuel cellsystem according to this mode of embodiment includes a fuel cell stack(not shown) configured as a lamination of a plurality of unit cells 2each serving as a fundamental unit for power generation byelectrochemical reactions, while the lamination has end flanges (notshown) fit on its both ends and connected at their peripheral parts bytie bolts (not shown). Each unit cell 2 comprises a membrane electrodeassembly 3 comprising a solid polymer electrolyte membrane 4, and an airelectrode 5 and a fuel electrode 6, with the solid polymer electrolytemembrane 4 in between, an air electrode side separator 7 disposed on theair electrode 5 side of the membrane electrode assembly 3, cooperatingwith the membrane electrode assembly 3 to have air channels definedtherebetween, and a fuel electrode side separator 9 disposed on asurface at the fuel electrode 6 side of the membrane electrode assembly3, cooperating with the membrane electrode assembly 3 to have fuel gaschannels 10 defined therebetween.

As the solid polymer electrolyte membrane 4 in unit cell 2, there may beemployed a film of perfluorocarbon polymer having sulfonate group (tradename: Nafion® 112 by Du Pont Co., U.S.A.), and the like. The membraneelectrode assembly 3 is configured with catalytic layers having platinumcatalyst supported by carbon, of which one is joined as the airelectrode 5 to either side of the solid polymer electrolyte membrane 4,and the other, as the fuel electrode 6 to the opposite side.

The air electrode side separator 7 and the fuel electrode side separator9 are configured as plate-shaped carbon or metal members, which have gaschannels and cooling water channels formed in surfaces thereof. The airchannels 8 are formed between the air electrode 5 and the air electrodeside separator 7, to supply the air electrode 5 with air as a reactiongas. The fuel gas channels 10 are formed between the fuel electrode 6and the fuel electrode side separator 9, to supply the fuel electrode 6with hydrogen as a reaction gas. The fuel gas channels 10 are adapted toserve as paths for moisture supplement by humidification of fuel gas,and the air channels 8 are adapted to serve as paths for removal ofproduced water, as well. Between the electrode 5, 6 and the separator 7,9, there may be interposed an adequate gas diffusion layer made of;e.g., carbon paper, unwoven carbon cloth, etc.

In each unit cell 2 of the solid polymer electrolyte type fuel cell 1configured as described, the air channels 8 and the fuel gas channels 10are respectively supplied with air and hydrogen gas, whereby air andhydrogen gas are fed to the air electrode 5 and the fuel electrode 6,respectively, causing reactions shown by formulas (B1) and (B2) below.At the fuel electrode:H₂→2H⁺+2e ⁻  (B1)At the air electrode:(½)O₂+2H⁺+2e ⁻→H₂O  (B2)

As illustrated in FIG. 3, with hydrogen gas fed to the fuel electrode 6,the reaction of formula (B1) develops, generating H⁺ (proton) and e⁻(electron). H⁺ is hydrated to move through the solid polymer electrolytemembrane 4 to the air electrode 5, where it reacts to e- and oxygen gasof air fed thereto, so that the reaction of formula (B2) develops,producing water. With an electromotive force then produced, electronsgenerated at the fuel electrode 6 are conducted to the air electrode 5,via an external circuit 17 shown in FIG. 3.

Such being the case, the reaction at the air electrode 5 appears as ageneration of water by four-electron reduction of molecular oxygen (O₂).This four-electron reduction of oxygen accompanies concurrent sidereactions that generate free radical, such as superoxide anion (O₂ ⁻) asa one-electron reduction body of oxygen, hydroperoxy radical (.OOH) as aconjugate acid of superoxide, hydrogen peroxide (H₂O₂) as a two-electronreduction body, and hydroxy radical (.OH) as a three-electron reductionbody. Generation mechanisms of those free radicals are considered to becomplex reactions by way of such elementary reaction processes as shownby formulas (B3) to (B7) below.O₂ +e ⁻→O₂ ⁻  (33)O₂ ⁻+H⁺→.OOH  (B4)O₂+2H⁺+2e ⁻→H₂O₂  (B5)H₂O₂+H⁺ +e ⁻+H₂O+.OH  (B6)H₂O₂→2.OH  (B7)

Generated free radicals are considered to be reduced finally to water,by way of such elementary reaction process as shown by formulas (B8) to(B10) below, where E^(o) is a standard redox potential given in terms ofNHE (normal hydrogen electrode)..OOH+H⁺ +e ⁻→H₂O₂,E^(o)=1.50 V  (B8)H₂O₂+2H⁺+2e ⁻→2H₂O,E^(o)=1.77 V  (B9).OH+H⁺ e ⁻→H₂O,E^(o)=2.85 V  (B10)

Now controversial is hydroxy radical that has a redox potential as highas 2.85V, and is strong in oxidizability. Hydroxy radical is mostreactive among free radicals, and has a very short life of one millionthsecond. As the oxidizability is strong, hydroxy radical reacts withanother molecule, unless it is promptly reduced Most controversial casesof oxidative degradation may have been caused by hydroxy radical.Generation of hydroxy radical is maintained by way of formulas (B3) to(B7) during power generation of fuel cell. Hydroperoxy radical andhydrogen peroxide, though weaker in oxidizability than hydroxy radical,return to water by ways of processes that may generate hydroxy radical.Like this, the generation of hydroxy radical continues semipermanently,so long as power is generated in a solid polymer electrolyte type fuelcell. The solid polymer electrolyte membrane may thus be deteriorated,unless the solid polymer electrolyte type fuel cell is continuouslysupplied with a compound that can inactivate hydroxy radical. Accordingto this mode of embodiment, the fuel cell system has an antioxidantsupply system 11 installed outside a fuel cell 1, for supplying anantioxidant to the fuel cell 1 of a proton-exchange membrane type, sothat even when power is generated at the fuel cell 1, continuouslygenerating active oxygen, the antioxidant can be supplied from outsidethe fuel cell, besides hydrogen ion or hydrogen acting as fuel, in amanner where it is uninvolved in the fuel cell reaction, allowing asuccessful inactivation and elimination of active oxygen, with aresultant prevention of deterioration of the solid polymer electrolytemembrane. Further, an efficient inactivation of active oxygen can bemaintained by the external supply of antioxidant, even in an environmentwhere the antioxidant tends to be oxidized.

In view of the generation of active oxygen that continuessemipermanently so long as power is generated in a solid polymerelectrolyte type fuel cell, it should be effective to continuouslysupply antioxidant as an antioxidant solution from air electrode or fuelelectrode. The supply may preferably be made from the fuel electrode 9side of the fuel cell 1. In the case of an antioxidant to be suppliedfrom the fuel electrode 9 side, the antioxidant supply system 11 isconfigured, as illustrated in FIG. 1, with e.g. an antioxidant solutiontank 12 having sealed therein a solution of the antioxidant, a liquidfeed pump 13 for feeding the antioxidant solution to the fuel electrode6 side of the fuel cell 1, an antioxidant solution line 14 forinterconnection between the antioxidant solution tank 12 and the liquidfeed pump 13, and an antioxidant supply line 15 for interconnectionbetween the liquid feed pump 13 and fuel gas channels 10. In the fuelcell 1, the air electrode side separator 7 and the fuel electrode sideseparator 9 have air channels 8 and fuel gas channels 10 formed insurfaces thereof for supplying air and hydrogen as reaction gases,respectively, as described. The reaction gases may be humidified bybubblers (not shown), and pass air channels 8 and fuel gas channels 10,as illustrated in FIG. 2, which is an exploded perspective view of aunit cell of a fuel cell constituting the fuel cell system. Theantioxidant solution is then fed by drive power of the liquid feed pump13, from the antioxidant solution tank 12 to a fuel gas channel 10, viaantioxidant solution line 14 and antioxidant supply line 15. Theantioxidant solution fed to fuel gas channel 10 is diffused in the solidpolymer electrolyte membrane 4, moving from the fuel electrode 6 side tothe air electrode 5 side, as illustrated in FIG. 3, which is a diagramillustrating transfer of materials in a membrane-electrode assemblyconstituting the unit cell. As a result, the antioxidant is uniformlydispersed within the air electrode 5, depending on a gradient ofconcentration.

Such being the case, it is desirable to supply an antioxidant as anantioxidant solution from the fuel electrode 6 side, which is because ofthe possibility of occurrence of active oxygen, such as hydroxy radical,that increases within a region near a three-phase interface at the airelectrode. As illustrated in FIG. 4 that is a schematic diagramillustrating a three-phase interface at an air electrode, there isplatinum residing as electrode catalyst, as well as oxygen in the air,in a region vicinal to the three-phase interface at the air electrode,which region thus constitutes an extremely oxidizable environment.Therefore, if an antioxidant is supplied from the air electrode side,the antioxidant itself may be oxidized and vanished at a three-phaseinterface at the air electrode, resulting in an inefficient inactivationof active oxygen, whether hydroxy radical is present or not.

The air channels 8 of the air electrode side separator 7 described serveas channels for removal of produced water, as well. Overfed and unusedantioxidant, as well as antioxidant having been changed to an oxidant byinactivation of active oxygen, is oxidized by catalyst on a three-phaseinterface, and changed to CO₂, H₂O, N₂, etc. which can thus bedischarged from a discharge line 16 shown in FIG. 1, together withproduced water. The antioxidant, as it has been changed by reaction withactive oxygen into an oxidant, unstable radical or peroxide, is thuskept from acting as an initiator of additional oxidation, causing adeterioration of electrolyte membrane.

Although, in preparation of an antioxidant solution, one may beunfastidious about a low solubility for a uniform dispersion in the airelectrode, the solubility to a solvent should be hung on as beingimportant. In an insoluble case, inhibited entry or exit of hydrogenradical causes an insufficient exhibition of the effect to inactivateactive oxygen. A solvent should thus be selected for dissolution ofantioxidant, using an organic solvent solely or in combination withwater, as a mixed solvent, as necessary. The use of an organic solventshould however be checked for no adverse affect to the performance ofpower generation. It therefore is desirable from the view point ofgeneration performance to make an aqueous solution, if the antioxidantis dissoluble.

The antioxidant may preferably be a hydrocarbon system compound composedof four elements, being carbon, oxygen, nitrogen, and hydrogen. Otherelements else than carbon, oxygen, nitrogen, and hydrogen may poisonplatinum in electrode, adversely affecting a power generationperformance of the fuel cell. Base metal elements may promote generationof hydroxy radical. To cover an application including oxidation in anddischarge from air electrode, the antioxidant may preferably be composedsimply of the four elements being carbon, oxygen, nitrogen, andhydrogen, as a hydrocarbon system compound to be decomposed into CO₂,H₂O, N₂, and the like. Hydroxy radical has a very high redox potentialso that, thermodynamically, most hydrocarbon compounds composed of theabove-noted four elements may act as a reductant on hydroxy radical.Kinetically, those compounds may be different in reducing ability. Inview of high reactivity of hydroxy radical, it is desirable for theantioxidant to be kinetically faster in reducing reaction It also isimportant to consider the stability of the oxidant the antioxidant is tobe oxidized to, that is, the compound to be obtained when it is oxidizedby active oxygen. If the oxidant of antioxidant is unstable, theoxidized substance may act as an initiator of new side reaction,promoting the deterioration of electrolyte membrane. As compoundskinetically relatively fast and chemically stable in the state ofoxidant, there may be taken, for example, secondary alcohol systemcompounds having hydroxyl group, such as isopropanol, 2-butanol, andcyclohexanol, aromatic series having hydroxyl group, such as phenols,phenol, cresol, picric acid, naphthol, and hydroquinone, an ether systemcompounds, such as dioxane, tetrahydrofuran, and benzyl methyl ether,and nitrogen-containing system compounds, such as propylamine,diethylamine, acetamide, aniline, and N-hydroxy system compound.

In selection of such compounds, the stability, durability, and heatresistance of compound are important. In particular, the stability anddurability of compound are most important for the inactivation of activeoxygen to be maintained to use a fuel cell over a long term. Preferably,hydrolysate of oxidant of antioxidant should also be chemically stable.For inactivation of active oxygen, it should be effective if theantioxidant supplied to the fuel electrode be kept stable till itsdischarge from the air electrode. On the other hand, antioxidant usedfor inactivation of active oxygen is discharged together with producedwater, as described, and for a long-term operation of the system, thehydrolysate of antioxidant may preferably be stable without generatingradicals. For the operating temperature of fuel cell to be within arange of 80 to 90° C. in normal run, and for the heat resistance ofelectrolyte membrane to be enhanced in future, the antioxidant maypreferably be stable in heat resistance up to a temperature about 120°C.

Preferably, the compound for inactivating active oxygen should be such acompound as having an oxidation potential of 2.85V or less that can beoxidized by hydroxy radical at least promptly. More preferably, notsimply being oxidizable, but should it also be redox-reversible by anoxidation-reduction cycle where, as it is oxidized, the oxidant isreduced, whereby it comes back to an original for The redox potentialmay preferably be greeter than 0.68V (NHE) and smaller than 1.77V (NHE).0.68V (NHE) is a potential where hydrogen peroxide acts as a reducingagent, and hence provision of an equivalent or higher potential allowsthe oxidant of the compound in concern to oxide hydrogen peroxide,returning to the original form. On the other hand, 1.77V (NHE) is apotential where hydrogen peroxide acts as an oxidizing agent, and forequivalent or higher oxidation potentials, the oxidant of the compoundmay act as a new oxidizing agent, causing oxidation of electrolytemembrane and the like.

For the oxidizability of compound to be decreased, the redox potentialmay preferably be 1.00V or less. A fluorine system film may be used asan electrolyte membrane. In this case, the potential where the fluorinesystem electrolyte membrane is to be oxidized ranges 2.5V or more, andwith the oxidizability of 1.77V, the electrolyte membrane will not beoxidized, causing no problem. A hydrocarbon system film may be used asan electrolyte membrane. In this case, the hydrocarbon systemelectrolyte membrane may be oxidized when the redox potential of addedcompound exceeds 1.00V. Substituting typical organic compounds therefor,benzene is to be oxidized at 2.00V, toluene is at 1.93V, and xylene isat 1.58V. Hydrocarbon system electrolyte membrane is thus oxidized at alower redox potential than fluorine system electrolyte membrane.Therefore, by setting the redox potential within a range of 1.00V orless, the electrolyte membrane can be kept from being oxidized, allowingfor a long service, even in the use of a hydrocarbon system film. It isnoted that the actual redox potential (RHE: real hydrogen electrode) mayvary, depending various conditions, such as pH and temperature, and theselection may preferably be made within a matching range.

The reversibility of redox is important for the following reasons. Forfuel cells, preventing oxidation, while generating power, needsconsideration to the electrolytic oxidation. A situation is nowsupposed, in which a compound employed as an antioxidative substance forreducing active oxygen to water is supplied to an electrolyte, from theside of an electrode. The antioxidative substance may then be oxidizedby electrolytic oxidation at the electrode, thus having an oxidizedstate to enter the electrolyte. In particular, for compounds under 1.23V(NHE) that is a theoretical voltage of the solid polymer electrolytetype fuel cell, the possibility of electrolytic oxidation on the way tothe electrolyte is great. For any compound, unless it has a reversibleredox-ability, the function as an antioxidative substance is lost whenthe compound is oxidized by electrolytic oxidation. If the compound hasa reversible redox-ability, it will be regenerated, by hydrogen peroxideas a reducing agent, for example, as a reductant that again functions asan antioxidative substance. From such point of view, as well, the amountof a compound to be supplied as antioxidant can be reduced, if thecompound has a reversible redox-ability. Moreover, in the use of anantioxidant that has a reversible redox-ability, the antioxidant may bepositively oxidized by electrolytic oxidation, to thereby implement amethod of inactivating hydrogen peroxide without detouring via hydroxyradicals, allowing for the more effective inactivation of active oxygen.

The antioxidant may preferably comprise a compound represented by ageneral formula (I) below

where R₁ and R₂ respectively denote elements of a set of arbitrarysubstituent groups mutually identical or different, and X denotes anoxygen atom or hydroxyl group. More preferably, R₁ and R₂ are combinedwith each other, to form a double bond, an aromatic ring, or anonaromatic ring.

Further, this antioxidant may preferably comprise an imide compoundrepresented by a general formula (II) below

where a ring Y₁ comprises any ring of a set of 5-membered to 12-memberedrings double-bonded, aromatic or nonaromatic.

The above-noted compound is supplied as an antioxidant to theproton-exchange membrane fuel cell, where it efficiently reduceshydroxyl radical to water, thereby suppressing a deterioration ofelectrolyte membrane, through an elementary process shown by formula(B11) below.>NOH+.OH→>NO.+H₂O  (B11)

The supply of hydrogen causes generation of N-oxyl radical (>NO.), whichdraws out radical hydrogen from hydrogen peroxide, to return to anoriginal form of hydroxyamine (>NOH), as shown by formula (B12) below.2(>NO.)+H₂O₂→2(>NOH)+O₂  (B12)

FIG. 5 shows N-hydroxyphthalimide (NHPI) as a typical example of thecompound having a hydroxy imide group, and phthalimide N-oxyl (PINO) asan oxidant of NHPI in which NHPI is changed in a radical form, andillustrates a mechanism in which hydroxyl radical as an active oxygen aswell as hydrogen peroxide is inactivated. As shown in FIG. 5, NHPI actsas a reducing agent on hydroxyl radical, generating PINO and water, andPINO reacts with hydrogen peroxide to return to NHPI. At this time, PINOacts as an oxidizing agent on hydrogen peroxide, inactivating hydrogenperoxide into oxygen. Like this, a redox cycle turns between NHPI andPINO, enabling the use as an antioxidant to be repeated many times,which allows inactivation of active oxygen over a long term, allowingfor implementation of a fuel cell system with maintained durability. Inaddition, as the redox cycle turns, the antioxidant having reducedhydroxyl radical will not constitute an initiator of additional sidereactions.

Further, the above-noted compound may preferably comprise an imidecompound represented by a general formula (III) below

where R₃ and R₄ respectively denote elements of a set of hydrogen atoms,halogen atoms, alkyl groups, aryl groups, cycloalkyl groups, hydroxylgroups, alkoxyl groups, carboxyl groups, alkoxycarbonyl groups, or acylgroups, mutually identical or different, X denotes an oxygen atom orhydroxyl group, and n denotes an integer within 1 to 3.

In the compound represented by the general formula (III), substituentsR₃ and R₄ may include iodine, bromine, chlorine, and fluorine as halogenatoms. The alkyl groups may include those linear chain or branch chainalkyl groups which have carbon numbers within a range of 1 to 10 ornear, for example, a methyl, an ethyl, a propyl an isopropyl a butyl, anisobutyl a sec-butyl a t-butyl, a pentyl a hexyl a heptyl an octyl and adecyl group, and the like. They may preferably have carbon numberswithin a range of 1 to 6 or near, or more preferably, be as lower alkylgroups as carbon numbers within a range of 1 to 4 or near.

The aryl groups may include, for example, a phenyl group, a naphthylgroup, etc. The cycloalkyl groups may include a cyclopentyl a cyclohexyland a cyclooctyl group, and the like. The alkoxy groups may include, forexample, a methoxy, an ethoxy, a propoxy, an isopropoxy, a butoxy, anisobutoxy, a t-butoxy, a pentyloxy, and a hexyloxy group, and the like,that have carbon numbers within a range of 1 to 10 or near, preferably,within a range of 1 to 6 or near, or more preferably, be as lower alkoxygroups as carbon numbers within a range of 1 to 4 or near.

The alkoxycarbonyl groups may include those alkoxycarbonyl groups whichhave carbon numbers of their alkoxy parts within a range of 1 to 10 ornear, for example, a methoxycarbonyl, an ethoxycarbonyl a propoxycarbonyl, an isopropoxy carbonyl a butoxycarbonyl an isobutoxycarbonyl at-butoxycarbonyl a pentyloxy carbonyl a hexyloxy carbonyl group, and thelike. They may preferably have carbon numbers of alkoxy parts within arange of 1 to 6 or near, or more preferably, be as lower alkoxycarbonylgroups as carbon numbers within a range of 1 to 4 or near.

The acyl groups may include those acyl groups which have carbon numberswithin a range of 1 to 6 or near, for example, a formyl an acetyl apropionyl a butyryl, an isobutyryl, a valeryl an isovaleryl, a pivaloylgroup, and the like.

The substituents R₃ and R₄ may be mutually identical or different In thecompound represented by the general formula (III), the substituents R₃and R₄ may be combined with each other to form a double bond, anaromatic ring, or a nonaromatic ring. Among them, the aromatic ring ornonaromatic ring may preferably be any one kind of ring of 5-membered to12-membered rings, or more preferably, of roughly 6-membered to10-membered rings, and these may be heterocycles or fused heterocycles,or preferably, hydrocarbon rings.

As such rings, there may be taken, for example: nonaromatic hydrocarbonrings, such as cycloalkane rings, e.g., cyclohexane ring, andcycloalkene rings, e.g. cyclohexene ring; nonaromatic bridging rings,such as bridging type hydrocarbon rings, e.g. five-norbornene ring, andaromatic rings, such as benzene rings and naphthalene rings. Those ringsmay have substituent groups.

Further, in compounds represented by the general formula (III), ascompounds to be more preferable in particular from view points of thecompound's stability, durability, and solubility to electrolytemembrane, there may be employed those in which R₃ and R₄ are mutuallycombined to form an aromatic or nonaromatic one of 5-membered to12-membered rings, or those in which R₃ and R₄ are mutually combined toprovide a cycloalkane ring with a substituent, a cycloalkene ring with asubstituent, or a bridging type hydrocarbon ring with a substituent.

In particular from the points of compound's stability, durability, andsolubility to electrolyte membrane, the compound represented by thegeneral formula (III) may preferably comprise a compound represented byone of general formulas (IVa) to (IVf) below

where R₃ to R₄ respectively denote elements of a set of hydrogen atoms,halogen atoms, alkyl groups, hydroxyl groups, alkoxyl groups, carboxylgroups, alkoxycarbonyl groups, acyl groups, nitro groups, cyano groups,or amino groups, mutually identical or different, and n denotes aninteger within 1 to 3.

The substituents R₃ to R₆ may include as alkyl groups, those of likealkyl groups to the before-mentioned alkyl groups, which have carbonnumbers within a range of 1 to 6 or near, as alkoxy groups, like alkoxygroups to the before-mentioned alkoxy groups, in particular those loweralkoxy groups which have carbon numbers within a range of 1 to 4 ornear, and as alkoxycarbonyl groups, like alkoxycarbonyl groups to thebefore-mentioned alkoxycarbonyl groups, in particular those loweralkoxycarbonyl groups whose alkoxy parts have carbon numbers within arange of 1 to 4 or near.

Substituents R₃ to R₆ may include, as acyl groups, like acyl groups tothe before-mentioned acyl groups, in particular those acyl groups whichhave carbon numbers within a range of 1 to 6 or near. As halogen atoms,there may be cited fluorine, chlorine, and bromine atoms. As forsubstituents R₃ to R₆ they may typically be elements of a set ofhydrogen atoms, lower alkyl groups having carbon numbers within a rangeof 1 to 4 or near, carboxyl groups, nitro groups, and halogen atoms, inmost cases.

In view of the availability, synthesis to be facile, and cost ofcompound, more desirable imide compounds may preferably comprise animide compound selective from a set of N-hydroxy succinic acid imide,N-hydroxy maleic acid imide, N-hydroxy hexahydrophthalic acid imide,N,N′-dihydroxycyclohexane tetracarboxylic acid imide,N-hydroxyphthalimide, N-hydroxy tetrabromophthalic acid imide, N-hydroxytetrachlorophthalic acid imide, N-hydroxy fatty acid imide, N-hydroxyhimic acid imide, N-hydroxy trimellitic acid imide, N,N′-dihydroxypyromellitic acid imide, and N,N′-dihydroxynaphthalene tetracarboxylicacid imide. This compound may be disposed as a coexisting catalyst inthe electrolyte membrane. Such an imide compound may be prepared by astandard imidizing reaction in which a correspondent acid anhydridereacts with hydroxylamine NH₂OH, whereby its acid anhydride radical hasan opened ring, which is closed for imidization.

The compound resented by the general formula (II) may comprise such acompound that has an N-substitution cyclic imide frame, as representedby a general formula (V) below

where X denotes an oxygen atom or hydroxyl group, R₁ to R₆ respectivelydenote elements of a set of hydrogen atoms, halogen atoms, alkyl groups,aryl groups, cycloalkyl groups, hydroxyl groups, alkoxy groups, carboxylgroups, substituent carbonyl groups, acyl groups, or acyloxy groups,mutually identical or different. At least two of R₁ to R₆ may becombined with each other to form a double bond, an aromatic ring, or anonaromatic ring. Of the rings, at least one may have an N-substitutioncyclic imide group.

In the N-substitution cyclic imide frame, both 5-membered ring and6-membered ring are hydrolyzable as shown by formulas (B13) and (B14)below, while the 6-membered ring is slower in hydrolysis, and higher inhydrolysis resistance, than the 5-membered ring.

Therefore, if the compound having the N-substitution cyclic imide frameis a cyclic imide of a 6-membered ring, this can be reused many times asa redox catalyst, thus allowing the consumption of catalyst to be themore reduced.

It is noted that the alkyl groups may include linear chain or branchchain alkyl groups of carbon numbers within a range of 1 to 10 or near,for example, a methyl, an ethyl, a propyl, an isopropyl, a butyl, anisobutyl, a sec-butyl, a t-butyl, a pentyl, a hexyl, a heptyl, an octyl,and a decyl group, and the like. They may preferably have carbon numberswithin a range of 1 to 6 or near, or more preferably, be as lower alkylgroups as carbon numbers within a range of 1 to 4 or near.

Further, the aryl groups may include a phenyl group, a naphthyl group,etc., and the cycloalkyl groups may include a cyclopentyl, a cyclohexyl,and a cyclooctyl group, and the like. The alkoxy groups may include, forexample, a methoxy, an ethoxy, a propoxy, an isopropoxy, a butoxy, anisobutoxy, a t-butoxy, a pentyloxy, and a hexyloxy group, and the like,having carbon numbers within a range of 1 to 10 or near, preferably,within a range of 1 to 6 or near, or more preferably, be as lower alkoxygroups as carbon numbers within a range of 1 to 4 or near.

The alkoxycarbonyl groups may include those alkoxycarbonyl groups whosealkoxy parts have carbon numbers within a range of 1 to 10 or near, forexample, a methoxycarbonyl, an ethoxycarbonyl, a propoxy carbonyl, anisopropoxy carbonyl, a butoxycarbonyl, an isobutoxycarbonyl, at-butoxycarbonyl, a pentyloxy carbonyl, and a hexyloxy carbonyl group,and the like. They may preferably have carbon numbers of their alkoxyparts within a range of 1 to 6 or near, or more preferably, be as loweralkoxycarbonyl groups as carbon numbers within a range of 1 to 4 ornear.

The acyl groups may include those acyl groups which have carbon numberswithin a range of 1 to 6 or near, for example, a formyl, an acetyl, apropionyl, a butyryl, an isobutyryl, a valeryl, an isovaleryl, and apivaloyl group, and the like.

Further, in the compound represented by the general formula (V), atleast two of R₁ to R₆ may preferably be combined with each other to forma double bond, or an aromatic or nonaromatic ring. Among them, thearomatic ring or the nonaromatic ring may preferably be any one kind ofring of 5-membered to 12-membered rings, or more preferably, about6-membered to 10-membered rings, while the ring may be heterocycles orfused heterocycles. As such rings, there may be taken, for example,nonaromatic hydrocarbon rings, such as cycloalkane rings, e.g.,cyclohexane ring, and cycloalkene rings, e.g. cyclohexene ring;nonaromatic bridging rings, such as bridging type hydrocarbon rings,e.g. five-norbornene ring, and aromatic rings, such as benzene rings andnaphthalene rings. It is noted that those rings may have a substituentgroup.

In view in particular of the compound's stability, durability, and thelike, the compound represented by the general formula (V) may preferablycomprise a compound represented by one of general formulas (VIa) and(VIb) below

where R₇ to R₁₂ respectively denote elements of a set of hydrogen atoms,alkyl groups, hydroxyl groups, alkoxyl groups, carboxyl groups,alkoxycarbonyl groups, acyl groups, nitro groups, cyano groups, or aminogroups, mutually identical or different.

The compound represented by one of the general formulas (V), (VIa) and(VIb) may preferably comprise at least one kind of imide compoundselective from a set of N-hydroxyglutaric acid imide,N-hydroxy-1,8-naphthalene dicarboxylic acid imide, N-hydroxy-1,8-decalindicarboxylic acid imide, N,N′-dihydroxy-1,8,4,5-naphthalenetetracarboxylic acid imide, N,N′-dihydroxy-1,8,4,5-decalintetracarboxylic acid imide, and N,N′,N″-trihydroxy isocyanuric acidimide.

The cyclic imide of 6-membered ring can be prepared by a standardimidizing reaction in which, for example, a correspondent acid anhydrideof 6-membered ring reacts with hydroxylamine NH₂OH, whereby its acidanhydride radical has an opened ring, which is closed for imidization.This cyclic imide of 6-membered ring is disposed for a coexistence inthe electrolyte membrane, like the cyclic imide of 5-membered ring,whereby elementary processes progress, as shown by formulas (B15) and(B16) below. And, simply upon an entry such as of hydroxyl radical orhydrogen peroxide into electrolyte membrane, the 6-membered imide ringsupplies hydrogen radical, which efficiently reduces hydrogen peroxide,suppressing oxidation of the electrolyte membrane.>NOH+.OH→>NO.+H₂O  (B15)2(>NOH)+H₂O₂→2(>NO.)+2H₂O  (B16)

The supply of hydrogen generates N-oxyl radical (>NO.), which draws outhydrogen ion from hydrogen element or hydrogen peroxide, and returns tothe original form of hydroxyimide (>NOH), as shown by formulas (B17) to(B19) below.2(>NO.)+H₂→2(>NOH)  (B17)>NO.+H⁺ +e ⁻→>NOH  (B18)2(>NO.)+H₂O₂→2(>NOH)+O₂  (B19)

FIG. 6 shows N-hydroxy glutaric acid imide (NHGI) as a typical exampleof the compound having a hydroxy imide group, and glutaric acid imideN-oxyl (GINO) as an oxidant of NHGI in which NHGI is changed in aradical form, and illustrates a mechanism in which a cycle tuns betweenhydroxy imide group of NHGI and N-oxyl radical of GINO, thereby havinghydroxy radical as well as hydrogen peroxide vanished over a long term.That is, NHGI acts as a reducing agent on hydroxyl radical or hydrogenperoxide, for reducing hydroxyl radical or hydrogen peroxide into water.On the other hand, GINO acts as an oxidizing agent on hydrogen peroxide,for oxidizing hydrogen peroxide into oxygen. Like this, a redox cycleturns between NHGI and GINO, concurrently vanishing hydroxy radical aswell as hydrogen peroxide. In the N-substitution cyclic imide frame, the6-membered ring is slower in hydrolysis, and higher in hydrolysisresistance, than the 5-membered ring, and hence, if the compound havingthe N-substitution cyclic imide frame is a cyclic imide of a 6-memberedring, this allows the consumption of catalyst to be the more reduced.

The compound represented by the general formula (I) may comprise acompound represented by a general formula (VII) below

where R₁₃ and R₁₄ each respectively denote an alkyl group, or an alkylgroup substituted in part by an arbitrary radical, wherein R₁₃ and R₁₄may be chained, ringed, or branched R₁₃ and R₁₄ may be mutually combinedto form a ring, and may include oxygen and nitrogen atoms. The compoundrepresented by the general formula (VII) may be continuously supplied,for inactivation of continuously generated active oxygen to suppress anoxidation of electrolyte membrane. In the compound represented by thegeneral formula (VII), substituents R₁₃ and R₁₄ may include linear chainor branch chain alkyl groups of carbon numbers within a range of 1 to 10or near, for example, a methyl, an ethyl a propyl, an isopropyl, a butylan isobutyl a sec-butyl a t-butyl a pentyl, a hexyl, a heptyl, an octyl,and a decyl group, and the like. They may preferably have carbon numberswithin a range of 1 to 6 or near, or more preferably, be as lower alkylgroups as carbon numbers within a range of 1 to 4 or near.

The compound represented by the general formula (VII) may preferablycomprise a compound represented by a general formula (VII) below

where R₁₃ to R₁₆ each respectively denote an alkyl group, or an alkylgroup substituted in part by an arbitrary radical, wherein R₁₃ to R₁₆may be chained, ringed, or branched. Among them, R₁₃ and R₁₄, or R₁₅ andR₁₆ may be mutually combined to form a ring, and they may include oxygenand nitrogen atoms. In the compound represented by the general formula(VII), substituents R₁₃ to R₁₆ may include linear chain or branch chainalkyl groups of carbon numbers within a range of 1 to 10 or near, forexample, a methyl, an ethyl, a propyl, an isopropyl, a butyl, anisobutyl, a sec-butyl, a t-butyl, a pentyl, a hexyl, a heptyl, an octyl,and a decyl group, and the like. They may preferably have carbon numberswithin a range of 1 to 6 or near, or more preferably, be as lower alkylgroups as carbon numbers within a range of 1 to 4 or near.

The compound represented by the general formula (VIII) may preferablycomprise a compound represented by a general formula (IX) below

where a ring Y₂ denotes a 5-membered or 6-membered ring formed by R₁₃and R₁₄ mutually combined. As such rings, there may be taken, forexample, nonaromatic hydrocarbon rings, such as cycloalkane rings, e.g.,cyclohexane ring, and cycloalkene rings, e.g. cyclohexene ring,nonaromatic bridging rings, such as bridging type hydrocarbon rings,e.g. five-norbornene ring, and aromatic rings, such as benzene rings andnaphthalene rings. It is noted that those rings may have a substituentgroup.

The compound represented by the general formula (IX) may preferablycomprise a compound represented by a general formula (X) below

where Z denotes a kind of substituent selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, amino groups, andsubstituent groups including a hydrogen atom. For Z being an alkylgroup, the an alkyl group may be substituted in part by an arbitraryradical, may be chained, ringed, or branched in part, and may includeoxygen and nitrogen atoms. For Z being an aryl group, the aryl group maybe substituted in part by an arbitrary radical, and may include oxygenand nitrogen atoms. The compound represented by the general formula (X)is hardly hydrolyzable, and may be continuously supplied, forinactivation of continuously generated active oxygen to suppress anoxidation of electrolyte membrane.

For substituent Z in the compound represented by the general formula(X), there may be taken alkyl groups, in particular those of like alkylgroups to the before-mentioned alkyl groups, which have carbon numberswithin a range of 1 to 6 or near, while as aryl groups, there may betaken phenyl groups and naphthyl groups. There may be taken alkoxygroups, in particular those of like alkoxy groups to thebefore-mentioned alkoxy groups, which have carbon numbers within a rangeof 1 to 6 or near, and as carboxyl groups, those carboxyl groups whichhave carbon numbers within a range of 1 to 4 or near, for example. Asalkoxycarbonyl groups, there may be taken those alkoxycarbonyl groupswhose alkoxy parts have carbon numbers within a range of 1 to 10 ornear, for example, a methoxycarbonyl, an ethoxycarbonyl, a propoxycarbonyl, an isopropoxy carbonyl, a butoxycarbonyl, anisobutoxycarbonyl, a t-butoxycarbonyl, a pentyloxy carbonyl, and ahexyloxy carbonyl group, and the like. They may preferably have carbonnumbers of alkoxy parts within a range 1 to 6 or near, or morepreferably, be as lower alkoxycarbonyl groups as carbon numbers within arange of 1 to 4 or near.

As an example of compound represented by the general formula (X), thereis now taken TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). FIG. 7 showschemical formulas of examples of compounds represented by the generalformula (X), with TEMPO inclusive. FIG. 7 (i) shows TEMPO as a compoundhaving a reversible redox-cycle, which finally inactivates activeoxygen.

The compound represented by the general formula (IX) may comprise acompound represented by a general formula (XI) below

where Z denotes a kind of substituent selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, amino groups, andsubstituent groups including a hydrogen atom. For Z being an alkylgroup, the an alkyl group may be substituted in part by an arbitraryradical, may be chained, ringed, or branched in part, and may includeoxygen and nitrogen atoms. For Z being an aryl group, the aryl group maybe substituted in part by an arbitrary radical, and may include oxygenand nitrogen atoms.

The compound represented by the general formula (IX) may comprise acompound represented by a general formula (XII) below

where Z denotes a kind of substituent selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, ammo groups, andsubstituent groups including a hydrogen atom. For Z being an alkylgroup, the an alkyl group may be substituted in part by an arbitraryradical, may be chained, ringed, or branched in part, and may includeoxygen and nitrogen atoms. For Z being an aryl group, the aryl group maybe substituted in part by an arbitrary radical, and may include oxygenand nitrogen atoms. Also these compounds are hardly hydrolysable likethat represented by the general formula (X), and may be continuouslysupplied, for inactivation of continuously generated active oxygen tosuppress an oxidation of electrolyte membrane. For compounds representedby the general formula (XI) or (XII), the substituents to be employedmay be like to the compound represented by the general formula (X).

Examples of compounds represented by the general formula (XI) or (XII)are shown in FIG. 8 to FIG. 10. As examples of compounds represented bythe general formula (XI) or (XII), PROXYL(2,2,5,5-tetraethylpyrrolidine-1-oxyl) and DOXYL(4,4-dimethyloxazolidine-3-oxyl) are now taken. Like TEMPO, thesecompounds also have a reversible redox cycle, and can serve forinactivation of active oxygen.

FIG. 11 shows a mechanism of oxidation and reduction by another exampleof compound employable in the fuel cell system according to this mode ofembodiment. For TEMPO, a typical example of the compound, its redoxcycle is illustrated as a mechanism in which hydroxyl radical as well ashydrogen peroxide is inactivated by TEMPO.

It is known that hydrogen peroxide acts as a reducing agent on asubstance whose redox potential is higher than hydrogen peroxide, and onthe other hand, as an oxidizing agent on a substance whose redoxpotential is lower than hydrogen peroxide, as in the formula (B9) and asin a formula (B20) below.H₂O₂→O₂+2H⁺+2e ⁻,E^(o)=0.68 V  (B20)

TEMPO is an N-hydroxy imide derivative that has a reversible redoxcycle, where performs oxidation and reduction through elementaryreaction processes with a redox potential of 0.81V, as shown by formulas(B21) and (B22) below.TEMPO⁺ +e ⁻→TEMPO,E^(o)=0.81 V  (B21)TEMPO→TEMPO⁺ +e ⁻,E^(o)=0.81 V  (B22)

TEMPO has a redox potential higher than the redox potential of hydrogenperoxide, and lower than the redox potential of hydroxy radical.Therefore, an N-oxyl radical of TEMPO, that is, a reductant acts as areducing agent on hydroxy radical, whereby it supplies an electron (e⁻)to hydroxy radical generated in the electrolyte membrane, which isthereby reduced to OH⁻.TEMPO+.OH→TEMPO⁺+OH⁻  (B22)

On the other hand, TEMPO⁺ being an oxidant acts as an oxidizing agent onhydrogen peroxide, i.e., performs oxidation of hydrogen peroxide, takingout hydrogen, so that hydrogen peroxide is oxidized to oxygen, wherebyTEMPO⁺ is changed to have a recovered form of reductant2TEMPO⁺+H₂O₂→2TEMPO+2H⁺+O₂  (B23)

After the recovery to a reductant, TEMPO again acts to reduce hydroxyradical. Like this, a redox cycle turns between reductant and oxidant ofTEMPO, which concurrently inactivates hydroxy radical as well ashydrogen peroxide, thus preventing oxidation of electrolyte membrane.

In a situation that TEMPO is supplied from the fuel electrode of fuelcell, part of TEMPO may be oxidized by electrolytic oxidation shown byformula (B22), whereby it may be changed to TEMPO⁺ being an oxidant, anddiffused in the electrolyte. In this case also, TEMPO, which has areversible redox cycle, cooperates with hydrogen peroxide acting as areducing agent, to recover the original reductant form of TEMPO, whichagain functions as an oxidizing agent that can reduce hydroxy radical.Unless the compound has a reversible redox cycle, its antioxidantfunction is lost when it has reduced hydroxy radical, so that it willnot function any more as an oxidizing agent However, in the case of acompound having a reversible redox cycle, the reversible redox cycleallows the function as an oxidizing agent to be kept to some extent.

Such being the case, according to this mode of embodiment, a fuel cellsystem is configured with a fuel cell having an electrode, and anantioxidant contacting the electrode, for inactivating active oxygen,and an antioxidant supply system for supplying the antioxidant from anair electrode side or a fuel electrode side of the fuel cell, therebyallowing the fuel cell system to be implemented for an ensuredinactivation and elimination of active oxygen.

In the fuel cell system according to this mode of embodiment, the fuelcell to be employed may well be any one of a hydrogen type, a directmethanol type, and a direct hydrocarbon type.

The fuel cell system according to this mode of embodiment may be mountedon a fuel cell vehicle, as an application thereof. The fuel cell vehicleis allowed to endure a continuous run over a long time, by mountingthereon the fuel cell system according to this mode of embodiment Thefuel cell system has applications thereof not limited to a fuel cellvehicle, and is applicable, for example, to a fuel cell cogenerationpower generating system, a fuel cell home electric appliance, a fuelcell portable device, a fuel cell transport machine, and the like.

(Solid Polymer Electrolyte)

Next, description is made of a solid polymer electrolyte according tothe present invention. According to this mode of embodiment, the solidpolymer electrolyte is characterized by a compound having a redox cycle,where it acts as a reducing agent in a range of potentials lower than aredox potential of hydroxy radical and as an oxidizing agent in a rangeof potentials higher than a redox potential where hydrogen peroxide actsas a reducing agent.

For a normal fuel cell, the reactions of entire system are as shown informulas (C1) and (C2) below.2H₂+O₂→2H₂O  (C1)H₂+O₂→H₂O₂  (C2)

The formula (C1) is a four-electron reduction of oxygen, and the formula(C2) is a two-electron reduction of oxygen. The reactions of formulas(C1) and (C2) concurrently progress as competitive reactions.

The formula (C1) is a sum of elementary reactions, being a positiveelectrode main reaction shown by a formula (C3) and a negative electrodereaction shown by a formula (C4). E^(o) denotes a standard redoxpotential (NHE).O₂+4H⁺+4e ⁻→2H₂O,E^(o)=1.23V  (C3)H₂−2H⁺+2e ⁻,E^(o)=0.00V  (C4)

Likewise, the formula (C2) is a sum of elementary reactions, being apositive electrode side reaction shown by a formula (C5) and thenegative electrode reaction shown by the formula (C4).O₂+2H⁺+2e ⁻→H₂O₂,E^(o)=0.68V  (C5)

Thermodynamically, a substance that has a higher redox potential acts asan oxidizing agent, and a substance that has a lower redox potentialacts as a reducing agent. Then, the reaction develops when ΔE^(o) of theentire system, i.e., the difference of redox potentials has a positivevalue.

Hydrogen peroxide generated in formula (C2) is considered to disappearby reactions shown by formulas (C6) to (C8) below.H₂O₂+H₂→2H₂O  (C6)H₂O₂+2H⁺+2e ⁻→2H₂O,E^(o)=1.77V  (C7)2H₂O₂→2H₂O+O₂  (C8)

The formula (C6) indicates a case in which hydrogen peroxide is reducedto water by H₂ having crossed over from the negative elective, via theelectrolyte membrane. The formula (C7) indicates a case in whichhydrogen peroxide receives hydrogen ion and electron at the positiveelectrode, whereby it is reduced to water. The formula (C8) indicates areaction in which two molecules of hydrogen peroxide react, eitheracting as an oxidizing agent and the other acting as a reducing agent,generating water and oxygen. It is known that hydrogen peroxide acts asa reducing agent on a substance that has a higher redox potential thanhydrogen peroxide, and as an oxidizing agent on a substance that has alower redox potential than hydrogen peroxide, as shown by the formula(C7) and a formula (C9).H₂O₂→O₂+2H⁺+2e ⁻,E^(o)=0.68V  (C9)

The formula (C6) is a sum of the elementary reaction shown by formula(C7) and the elementary reaction shown by formula (C4). The formula (C8)is a sum of the elementary reaction shown by formula (C7) and theelementary reaction shown by formula (C9).

It is now supposed that fuel cell is contaminated with Fe²⁺ ion.Hydrogen peroxide generated at the positive electrode is changed tohydroxy radical by a Fenton reaction by Fe²⁺ acting as a catalystFe²⁺+H₂O₂→Fe³⁺+OH⁻+.OH  (C10)

This hydroxyl radical has a redox potential of 2.85V, and has a veryhigh oxidizability. Hydroxyl radical oxidizes hydrogen ion as shown by aformula (C11), whereby it disappears..OH+H⁺ +e ⁻→H₂O,E^(o)=2.85V  (C11)

However, supply of hydrogen (hydrogen ion) may be short, upon a stoppedstartup of the fuel cell or the like, when hydroxyl radical that has avery high oxidizability may cut an C—F bond of the Nafion® membranewhich inherently is kept from oxidation. According to this mode ofembodiment, the solid polymer electrolyte involves a compound that has aredox cycle, where it acts as a reducing agent in a range of potentialslower than a redox potential of hydroxy radical, and as an oxidizingagent in a range of potentials higher than a redox potential wherehydrogen peroxide acts as a reducing agent. This compound acts as acatalyst, and decomposes hydrogen peroxide and hydroxy radical as activeoxygen. Therefore, hydroxyl radical is vanished by catalysis of thecompound contained in the electrolyte membrane, without oxidizing theelectrolyte membrane. Further, after the decomposition of active oxygen,this catalytic compound returns to an original reductant form by a redoxcycle of the compound, allowing a repeated use many times. This enablesimplementation of a solid polymer electrolyte with a maintaineddurability.

As an example of this compound acting as a catalyst, here is takenN-hydroxyphthalimide (NHPI) that has a reversible redox cycle, anddescription is made of a case in which NHPI is introduced into a fuelcell reaction system NHPI becomes PINO (phthalimide N-oxyl), whenoxidize NHPI and PINO have a redox potential of 1.34VNHPI→PINO+H⁺ +e ⁻,E^(o)=1.34V  (C12)PINO+H⁺ +e ⁻→NHPI,E^(o)=1.34V  (C13)

As shown by formulas (C7) and (C11), hydrogen peroxide and hydroxylradical generated by Fenton reaction have higher redox potentials thanNHPI. Therefore, under presence of NHPI, hydrogen peroxide and hydroxylradical is reduced to water by a reducing action of the NHPI as areducing agent In this case, as shown by formulas (C14) and (C15), NHPIis oxidized to PINO by hydroxyl radical or hydrogen peroxide..OH+NHPI→H₂O+PINO  (C14)H₂O₂+2NHPI→2H₂O+2PINO  (C15)

The formula (C14) is a sum of the elementary reaction shown by formula(C11) and the elementary reaction shown by formula (C12). The formula(C15) is a sum of the elementary reaction shown by formula (C7) and theelementary reaction shown by formula (C13).

PINO as an oxidant form of NHPI acts as an oxidizing agent, oxidizinghydrogen ion, hydrogen, or hydrogen peroxide, whereby it is reduced toNHPI, as shown by formulas (C16), (C17), and (C13).2PINO+H₂→2NHPI  (C16)PINO+H⁺ +e ⁻→NHPI,E^(o)=1.34V  (C13)2PINO+H₂O₂→2NHPI+O₂  (C17)

The formula (C16) is a sum of the elementary reaction shown by formula(C13) and the elementary reaction shown by formula (C4). The formula(C17) is a sum of the elementary reaction shown by formula (C13) and theelementary reaction shown by formula (C9). PINO has a redox potential of1.34V, which is thermodynamically not so high as to cut C—F bond ofNafion® membrane. Therefore, the electrolyte membrane will not beoxidized by PINO.

FIG. 12 shows redox potentials of hydroxy radical, oxygen, hydrogenperoxide, hydrogen, NHPI, and PINO. In this figure, the right columngives oxidation half reaction formulas of reducing agents, and the leftcolumn, reduction half reaction formulas of oxidizing agents. The axisof ordinate represents a standard redox potential, which is increased asit extends upwards. That is, the difficulty of oxidation is increased,as it is located upper. The half reaction formulas are followed byparenthesized values, which are standard redox potentials of compoundsacting as an oxidizing agent or a reducing agent. The oxidationreduction potential may be influenced by pH, temperate, etc., and isgiven, in FIG. 12, in terms of a standard redox potential corrected tothe normal hydrogen electrode (NHE).

Like this, according to this mode of embodiment, the solid polymerelectrolyte involves a compound that has a redox cycle, where it acts asa reducing agent in a range of potentials lower than a redox potentialof hydroxy radical, and as an oxidizing agent in a range of potentialshigher than a redox potential where hydrogen peroxide acts as a reducingagent Therefore, this compound acts as a catalyst, and decomposes activeoxygen such as hydrogen peroxide and hydroxy radical. Therefore, C—Fbond of electrolyte membrane is kept from being cut by hydroxyl radicalor hydrogen peroxide, so that deterioration of electrolyte membrane canbe prevented. Further, after the decomposition of active oxygen, thiscatalytic compound returns to an original reductant form by a redoxcycle of the compound, allowing a repeated use many times. This enablesimplementation of a solid polymer electrolyte with a maintaineddurability.

Although simply NHPI is taken as an example of a compound acting as acatalyst, this compound may preferably have a standard redox potentialwithin a range of 0.68V to 2.85V, when aiming at vanishing hydroxylradical only. When aiming at vanishing both hydroxyl radical andhydrogen peroxide, the compound should act as a reducing agent in arange of potentials lower than a redox potential where hydrogen peroxideacts as an oxidizing agent, and as an oxidizing agent in a range ofpotentials higher than a redox potential where hydrogen peroxide acts asa reducing agent, and may preferably have a standard redox potentialwithin a range of 0.68V to 1.77V.

Further, this compound may be preferable if its reductant and oxidantare relatively stable compounds. It is noted that the actual redoxpotential (RHE) may vary, depending various conditions, such as pH andtemperature, and the selection may preferably be made within a matchingrange. However, in view of poisoning to platinum used in the electrode,the compound to be employed for the solid polymer electrolyte accordingto this mode of embodiment may preferably comprise an organic compoundcomposed simply of carbon, hydrogen, oxygen, and nitrogen. Like this,the solid polymer electrolyte according to this mode of embodiment cansuppress deterioration of electrolyte membrane, allowing forimplementation of a solid polymer electrolyte with a maintaineddurability.

The above-noted compound may preferably comprise a compound representedby a general formula (I) below

where R₁ and R₂ respectively denote elements of a set of arbitrarysubstituent groups mutually identical or different, and X denotes anoxygen atom or hydroxyl group. More preferably, R₁ and R₂ should becombined with each other, to form a double bond, an aromatic ring, or anonaromatic ring.

Further, this compound may preferably comprise an imide compoundrepresented by a general formula (II) below

where a ring Y₁ comprises any ring of a set of 5-membered to 12-memberedrings double-bonded, or aromatic or nonaromatic.

The above-noted compound coexists in the electrolyte membrane, wherebyelementary reactions develop as shown by formulas (C18) and (C19) below.And, simply when hydroxy radical as well as hydrogen peroxide invadesthe electrolyte membrane, N-oxyl radical (>NO.) is provided by thatcompound, which efficiently reduces hydroxy radical as well as hydrogenperoxide into water, suppressing oxidation of electrolyte membrane.>NOH+.OH→>NO.+H₂O  (C18)2(>NOH)+H₂O₂→2(>NO.)+2H₂O  (C19)

Further, supply of hydrogen generates N-oxyl radical, which draws outradical hydrogen from hydrogen or hydrogen peroxide by elementaryreactions shown by formulas (C20) to (C22), and returns to the originalform of hydroxyamine (>NOH).2(>NO.)+H₂→2(>NOH)  (C20)>NO.+H⁺ +e ⁻→>NOH  (C21)2(>NO.)+H₂O₂→2(>NOH)+O₂  (C22)

FIG. 5 shows N-hydroxyphthalimide (NHPI) as a typical example of thecompound having a hydroxy imide group, and phthal acid imide N-oxyl(PINO) as an oxidized form of NHPI in which NHPI is changed to aradical, and illustrates a mechanism in which a cycle turns betweenhydroxy imide group of NHPI and N-oxyl radical of PINO, thereby causinghydroxy radical as well as hydrogen peroxide to be vanished over a longterm. That is, NHPI acts as a reducing agent on hydroxyl radical orhydrogen peroxide, reducing hydroxyl radical or hydrogen peroxide towater. On the other hand, PINO acts as an oxidizing agent on hydrogenperoxide, oxidizing hydrogen peroxide to oxygen Like this, a redox cycleturns between NHPI and PINO, concurrently vanishing hydroxy radical aswell as hydrogen peroxide.

Further, for the compound involved in the solid polymer electrolyteaccording to this mode of embodiment, vanishment of hydrogen peroxide bythe reduction and oxidation is associated with entry and exit ofhydrogen radical (hydrogen atom). By the entry and exit of hydrogenradical, hydroxyl radical generated upon decomposition of hydrogenperoxide is vanished even when residing in a place distant from thatcompound. FIG. 13 illustrates this mechanism. As an example of suchcompound, NHPI is assumed in use, which gives rise to a Grotthussmechanism that transfers hydrogen radical by exchange between NHPI andwater molecules, as illustrated in FIG. 13. As a reducing reactionprogresses, supplying hydrogen radical to water molecules as describedabove, the catalytic reaction develops, vanishing hydroxy radical,allowing for vanishment of hydroxy radical even if hydroxy radical isdistant from NHPI. Therefore, over a wide range in the electrolytemembrane, hydroxy radical residing there can be vanished.

In selection of a compound with such catalysis, the stability,durability, heat resistance, and solubility to electrolyte membrane ofthe compound are important. In particular, the stability and durabilityof the compound are most important for the fuel cell to be employed overa long term, as well as in view of the effect of active oxygenvanishment to be kept. For the operating temperature of fuel cell to bewithin a range of 80 to 90° C. in normal run, and for the heatresistance of electrolyte membrane to be enhanced in future, theelectrolyte membrane should have a sufficient heat resistance to bedurable even under temperatures about 120° C. Although, one may beunfastidious about a low solubility for a uniform dissolution of thiscompound to the electrolyte membrane, the solubility to water should behung on as being important. If insoluble to water, it may be depositedin the electrolyte membrane, inhibiting entry and exit of hydrogenradical, thus losing the catalytic function, that is, the effect ofvanishing active oxygen such as hydroxy radical, hydrogen oxide, etc.

Further, the above-noted compound may preferably comprise an imidecompound represented by a general formula (III) below

where R₃ and R₄ respectively denote elements of a set of hydrogen atoms,halogen atoms, alkyl groups, aryl groups, cycloalkyl groups, hydroxylgroups, alkoxyl groups, carboxyl groups, alkoxycarbonyl groups, or acylgroups, mutually identical or different, X denotes an oxygen atom orhydroxyl group, and n denotes an integer within 1 to 3.

Further, in the compound represented by the general formula (III),substituents R₃ and R₄ may include iodine, bromine, chlorine, andfluorine as halogen atoms. The alkyl groups may include those linearchain or branch chain alkyl groups which have carbon numbers within arange of 1 to 10 or near, for example, a methyl, an ethyl, a propyl, anisopropyl, a butyl, an isobutyl, a sec-butyl, a t-butyl, a pentyl, ahexyl, a heptyl, an octyl, and a decyl group, and the like. They maypreferably have carbon numbers within a range of 1 to 6 or near, or morepreferably, be as lower alkyl groups as carbon numbers within a range of1 to 4 or near.

The aryl groups may include, for example, a phenyl group, a naphthylgroup, etc. The cycloalkyl groups may include, for example, acyclopentyl, a cyclohexyl, and a cyclooctyl group, and the like. Thealkoxy groups may include, for example, a methoxy, an ethoxy, a propoxy,an isopropoxy, a butoxy, an isobutoxy, a t-butoxy, a pentyloxy, and ahexyloxy group, and the like, that have carbon numbers within a range of1 to 10 or near, preferably, within a range of 1 to 6 or near, or morepreferably, be as lower alkoxy groups as carbon numbers within a rangeof 1 to 4 or near.

The alkoxycarbonyl groups may include those alkoxycarbonyl groups whichhave carbon numbers of their alkoxy parts within a range of 1 to 10 ornear, for example, a methoxycarbonyl, an ethoxycarbonyl, a propoxycarbonyl, an isopropoxy carbonyl, a butoxycarbonyl, anisobutoxycarbonyl, a t-butoxycarbonyl, a pentyloxy carbonyl, a hexyloxycarbonyl group, and the like. They may preferably have carbon numbers ofalkoxy parts within a range of 1 to 6 or near, or more preferably, be aslower alkoxycarbonyl groups as carbon numbers within a range of 1 to 4or near.

The acyl groups may include those acyl groups which have carbon numberswithin a range of 1 to 6 or near, for example, a formyl, an acetyl, apropionyl, a butyryl, an isobutyryl, a valeryl, an isovaleryl, apivaloyl group, and the like.

The substituents R₃ and R₄ may be mutually identical or different In thecompound represented by the general formula (III), the substituents R₃and R₄ may be combined with each other to form a double bond, anaromatic ring, or a nonaromatic ring. Among them, the aromatic ring ornonaromatic ring may preferably be any one kind of ring of 5-membered to12-membered rings, or more preferably, of roughly 6-membered to10-membered rings, and these may be heterocycles or fused heterocycles,or preferably, hydrocarbon rings.

As such rings, there may be taken, for example: nonaromatic hydrocarbonrings, such as cycloalkane rings, e.g., cyclohexane ring, andcycloalkene rings, e.g. cyclohexene ring; nonaromatic bridging rings,such as bridging type hydrocarbon rings, e.g. five-norbornene ring, andaromatic rings, such as benzene rings and naphthalene rings. Those ringsmay have substituent groups.

In particular from the points of compound's stability, durability, andsolubility to electrolyte membrane, the compound represented by thegeneral formula (III) may preferably comprise a compound represented byone of general formulas (IVa) to (IVf) below

where R₃ to R₆ respectively denote elements of a set of hydrogen atoms,halogen atoms, alkyl groups, hydroxyl groups, alkoxyl groups, carboxylgroups, alkoxycarbonyl groups, acyl groups, nitro groups, cyano groups,or amino groups, mutually identical or different, and n denotes aninteger within 1 to 3.

The substituents R₃ to R₆ may include as allyl groups, those of likealkyl groups to the before-mentioned alkyl groups, which have carbonnumbers within a range of 1 to 6 or near, as alkoxy groups, like alkoxygroups to the before-mentioned alkoxy groups, in particular those loweralkoxy groups which have carbon numbers within a range of 1 to 4 ornear, and as alkoxycarbonyl groups, like alkoxycarbonyl groups to thebefore-mentioned alkoxycarbonyl groups, in particular those loweralkoxycarbonyl groups whose alkoxy parts have carbon numbers within arange of 1 to 4 or near.

Substituents R₃ to R₆ may include, as acyl groups, like acyl groups tothe before-mentioned acyl groups, in particular those acyl groups whichhave carbon numbers within a range of 1 to 6 or near. As halogen atoms,there may be cited fluorine, chlorine, and bromine atoms. As forsubstituents R₃ to R₆, they may typically be elements of a set ofhydrogen atoms, lower alkyl groups having carbon numbers within a rangeof 1 to 4 or near, carboxyl groups, nitro groups, and halogen atoms, inmost cases.

In view of the availability, synthesis to be facile, and cost ofcompound, more desirable imide compounds may preferably comprise animide compound selective from a set of N-hydroxy succinic acid imide,N-hydroxy maleic acid imide, N-hydroxy hexahydrophthalic acid imide,N,N′-dihydroxycyclohexane tetracarboxylic acid imide,N-hydroxyphthalimide, N-hydroxy tetrabromophthalic acid imide, N-hydroxytetrachlorophthalic acid imide, N-hydroxy fatty acid imide, N-hydroxyhimic acid imide, N-hydroxy trimellitic acid imide, N,N′-dihydroxypyromellitic acid imide, and N,N′-dihydroxynaphthalene tetracarboxylicacid imide. This compound may be disposed as a coexisting catalyst inthe electrolyte membrane.

Such an imide compound may be prepared by a standard imidizing reactionin which a corespondent acid anhydride reacts with hydroxylamine NH₂OH,whereby its acid anhydride radical has an opened ring, which is closedfor imidization.

The compound represented by the general formula (II) may comprise such acompound that has an N-substitution cyclic imide frame, as representedby a general formula (V) below

where X denotes an oxygen atom or hydroxyl group, R₁ to R₆ respectivelydenote elements of a set of hydrogen atoms, halogen atoms, alkyl groups,aryl groups, cycloalkyl groups, hydroxyl groups, alkoxy groups, carboxylgroups, substituent carbonyl groups, acyl groups, or acyloxy groups,mutually identical or different. At least two of R₁ to R₆ may becombined with each other to form a double bond or an aromatic ring or anonaromatic ring. Of the rings, at least one may have an N-substitutioncyclic imide group.

In compounds that have an N-substitution cyclic imide frame, both5-membered ring and 6-membered ring are hydrolyzable as shown byformulas (C23) and (C24) below, while the 6-membered ring is slower inhydrolysis, and higher in hydrolysis resistance, than the 5-memberedring.

Therefore, if the compound having the N-substitution cyclic inside frameis a cyclic imide of a 6-membered ring, this can be reused many times asa catalyst, thus allowing the consumption of catalyst to be the morereduced.

The alkyl groups may include linear chain or branch chain alkyl groupsof carbon numbers within a range of 1 to 10 or near, for example, amethyl, an ethyl, a propyl, an isopropyl, a butyl, an isobutyl, asec-butyl, a t-butyl, a pentyl a hexyl, a heptyl, an octyl, and a decylgroup, and the like. They may preferably have carbon numbers within arange of 1 to 6 or near, or more preferably, be as lower alkyl groups ascarbon numbers within a range of 1 to 4 or near.

The aryl groups may include, for example, a phenyl group, a naphthylgroup, etc., and the cycloalkyl groups may include, for example, acyclopentyl, a cyclohexyl, and a cyclooctyl group, and the like. Thealkoxy groups may include, for example, a methoxy, an ethoxy, a propoxy,an isopropoxy, a butoxy, an isobutoxy, a t-butoxy, a pentyloxy, and ahexyloxy group, and the like, having carbon numbers within a range of 1to 10 or near, preferably, within a range of 1 to 6 or near, or morepreferably, be as lower alkoxy groups as carbon numbers within a rangeof 1 to 4 or near.

The alkoxycarbonyl groups may include those alkoxycarbonyl groups whosealkoxy parts have carbon numbers within a range of 1 to 10 or near, forexample, a methoxycarbonyl, an ethoxycarbonyl, a propoxy carbonyl, anisopropoxy carbonyl, a butoxycarbonyl, an isobutoxycarbonyl, at-butoxycarbonyl, a pentyloxy carbonyl, and a hexyloxy carbonyl group,and the like. They may preferably have carbon numbers of their alkoxyparts within a range of 1 to 6 or near, or more preferably, be as loweralkoxycarbonyl groups as carbon numbers within a range of 1 to 4 ornear.

The acyl groups may include those acyl groups which have carbon numberswithin a range of 1 to 6 or near, for example, a formyl, an acetyl, apropionyl, a butyryl, an isobutyryl, a valeryl, an isovaleryl, and apivaloyl group, and the like.

Further, in the compound represented by the general formula (V), atleast two of R₁ to R₆ may preferably be combined with each other to forma double bond or an aromatic or nonaromatic ring. Among them, thearomatic ring or the nonaromatic ring may preferably be any one kind ofring of 5-membered to 12-membered rings, or more preferably, about6-membered to 10-membered rings, while the ring may be heterocycles orfused heterocycles. As such rings, there may be taken, for example,nonaromatic hydrocarbon rings, such as cycloalkane rings, e.g.,cyclohexane ring, and cycloalkene rings, e.g. cyclohexene ring;nonaromatic bridging rings, such as bridging type hydrocarbon rings,e.g. five-norbornene ring, and aromatic rings, such as benzene rings andnaphthalene rings. It is noted that those rings may have a substituentgroup.

As an example of the compound represented by the general formula (V),there is now taken N-hydroxyglutaric acid imide (NHGI) of which R₁ to R₆are all hydrogen atoms and which has a reversible redox cycle, to showthe case in which NHGI is introduced in a fuel cell reaction y NHGIbecomes glutaric acid imide N-oxyl (GINO) when oxidized. NHGI and GINOhave a redox potential of 1.39V.

NHGI is oxidized, and changed to glutaric acid imide N-oxyl (GINO). NHGIand GINO have the redox potential 1.39V.NHGI→GINO+H⁺ +e ⁻,E^(o)=1.39V  (C25)GINO+H⁺ +e ⁻→NHGI,E^(o)=1.39V  (C26)

As shown in formulas (C7) and (C11), hydrogen peroxide, as well ashydroxyl radical generated by Fenton reaction, has a redox potentialhigher than the redox potential of NHGI. Therefore, under presence ofNHGI, hydrogen peroxide or hydroxyl radical is reduced to water bycatalysis of NHGI. In this case, NHGI acts as a reducing agent, and thisNHGI is oxidized to GINO by hydroxyl radical, as well as by hydrogenperoxide, as shown by formulas (C27) and (C28) below..OH+NHGI→H₂O+GINO  (C27)H₂O₂+2NHGI→2H₂O+2GINO  (C28)

The formula (C27) is a sum of the elementary reaction shown by formula(C11) and the elementary reaction shown by formula (C25). The formula(C28) is a sum of the elementary reaction shown by formula (C7) and theelementary reaction shown by formula (C26).

GINO, which is an oxidized form of NHGI, acts as an oxidizing agent foroxidation of hydrogen ion, hydrogen, or hydrogen peroxide, whereby it isreduced to NHGI, as shown by formulas (C29), (30), and (C26) below.2GINO+H₂→2NHGI  (C29)GINO+H⁺ e ⁻→NHGI,E^(o)=1.39V  (C26)2GINO+H₂O₂→2NHGI+O₂  (C30)

The formula (C29) is a sum of the elementary reaction shown by formula(C26) and the elementary reaction shown by formula (C4). The formula(C30) is a sum of the elementary reaction shown by formula (C26) and theelementary reaction shown by formula (C9). GINO's redox potential is1.39V. Thermodynamically, this is not so high as to cut C—F bond ofNafion® film Therefore, the electrolyte membrane will not be oxidized byGINO.

Such being the case, according to this mode of embodiment, a solidpolymer electrolyte has such a compound that has an N-substitute cyclicimide frame of 6-membered ring having a redox cycle, where it acts as areducing agent in a range of potentials lower than a redox potential ofhydroxy radical, and as an oxidizing agent in a range of potentialshigher than a redox potential where hydrogen peroxide acts as a reducingagent, which compound acts as a catalyst, decomposing active oxygen suchas hydroxy radical and hydrogen peroxide. Therefore, C—F bond of theelectrolyte membrane can not be cut by hydroxy radical or hydrogenperoxide, so that deterioration of the electrolyte membrane ispreventive. Further, after decomposition of active oxygen, this compoundreturns to an original form by the redox cycle of compound, and can bereused many times. It therefore is possible to implement an electrolytefor proton-exchange membrane fuel cells with a maintained durability.

In view in particular of the compound's stability, durability, andsolubility to the electrolyte membrane, the compound represented by thegeneral formula (V) may preferably comprise a compound represented byone of general formulas (VIa) and (VIb) below

where R₇ to R₁₂ respectively denote elements of a set of hydrogen atoms,alkyl groups, hydroxyl groups, alkoxyl groups, carboxyl groups,alkoxycarbonyl groups, acyl groups, nitro groups, cyano groups, or ammogroups, mutually identical or different.

Further, the compound represented by one of the general formulas (V),(VIa) and (VIb) may preferably comprise at least one kind of imidecompound selective from a set of N-hydroxyglutaric acid imide,N-hydroxy-1,8-naphthalene dicarboxylic acid imide, N-hydroxy-1,8-decalindicarboxylic acid imide, N,N′-dihydroxy-1,8,4,5-naphthalenetetracarboxylic acid imide, N,N′-dihydroxy-1,8,4,5-decalintetracarboxylic acid imide, and N,N′,N″-trihydroxy isocyanuric acidimide.

The cyclic imide of 6-membered ring can be prepared by a standardimidizing reaction in which, for example, a correspondent acid anhydrideof 6-membered ring reacts with hydroxylamine NH₂OH, whereby its acidanhydride radical has an opened ring, which is closed for imidization.This cyclic imide of 6-membered ring is disposed for a coexistence inthe electrolyte membrane, like the cyclic imide of 5-membered ring,whereby elementary processes progress, as shown by formulas (C25) and(C26) below. And, simply upon an entry such as of hydroxyl radical orhydrogen peroxide into electrolyte membrane, the 6-membered imide ringsupplies hydrogen radical, which efficiently reduces hydrogen peroxide,suppressing oxidation of the electrolyte membrane.>NOH+.OH→NO.+H₂O  (C31)2(>NOH)+H₂O₂→2(>NO.)+2H₂O  (C32)

The supply of hydrogen generates N-oxyl radical (>NO.), which draws outhydrogen ion from hydrogen element or hydrogen peroxide, and returns tothe original form of hydroxyimide (>NOH), as shown by formulas (C33) to(C35) below.>2(>NO.)+H₂→2(>NOH)  (C33)>NO.+H⁺ +e ⁻→>NOH  (C34)2(>NO.)+H₂O₂→2(>NOH)+O₂  (C35)

FIG. 6 shows N-hydroxy glutaric acid imide (NHGI) as a typical exampleof the compound having a hydroxy imide group, and glutaric acid imideN-oxyl (GINO) as an oxidant of NHGI in which NHGI is changed in aradical form, and illustrates a mechanism in which a cycle turns betweenhydroxy imide group of NHGI and N-oxyl radical of GINO, thereby havinghydroxy radical as well as hydrogen peroxide vanished over a long term.That is, NHGI acts as a reducing agent on hydroxyl radical or hydrogenperoxide, for reducing hydroxyl radical or hydrogen peroxide into water.On the other hand, GINO acts as an oxidizing agent on hydrogen peroxide,for oxidizing hydrogen peroxide into oxygen. Like this, a redox cycleturns between NHGI and GINO, concurrently vanishing hydroxy radical aswell as hydrogen peroxide.

Further, for the imide compound of six-membered ring involved in thesolid polymer electrolyte according to this mode of embodiment,vanishment of hydrogen peroxide by the reduction and oxidation isassociated with entry and exit of hydrogen radical (hydrogen atom). Bythe entry and exit of hydrogen radical, hydroxy radical generated upondecomposition of hydrogen peroxide is vanished even when residing in aplace distant from that compound. This mechanism is like that shown inFIG. 13, there being constituted a Grotthuss mechanism that transfershydrogen radical by exchange between NHPI and water molecules.Therefore, even if hydroxy radical is distant from NHPI catalyticreaction develops, vanishing hydroxy radical, allowing for vanishment ofhydroxy radical even if. As a result, over a wide range in theelectrolyte membrane, hydroxy radical residing there can be vanished.

Like this, the solid polymer electrolyte according to this mode ofembodiment involves a compound that has a redox cycle, where it acts asa reducing agent in a range of potentials lower than a redox potentialof hydroxy radical, and as an oxidizing agent in a range of potentialshigher than a redox potential where hydrogen peroxide acts as a reducingagent. Therefore, this compound acts as a catalyst, and decomposesactive oxygen. Therefore, C—F bond of electrolyte membrane is kept frombeing cut by hydroxyl radical or hydrogen peroxide, so thatdeterioration of electrolyte membrane can be prevented. Further, afterthe decomposition of active oxygen, this catalytic compound returns toan original reductant form by a redox cycle of the compound, allowing arepeated use many times. This enables implementation of a solid polymerelectrolyte with a maintained durability. Further, in the case thiscompound is a cyclic imide of six-membered ring, it is slower inhydrolysis in comparison with the case of five-membered ring, and has ahigher resistance to hydrolysis. Therefore, in the case of cyclic imideof six-membered ring, the durability is still more maintained.

The above-noted compound may preferably have a redox potential within arange of 0.68V to 1.00V. The four-electron reduction of oxygenaccompanies side reactions that generate active oxygen. Such activeoxygen involves superoxide anion (O₂) as a one-electron reductant ofoxygen, hydroperoxy radical (.OOH) as a conjugate acid of superoxide,hydrogen peroxide (H₂O₂) as a two-electron reductant, and hydroxyradical (.OH) as a three-electron reductant Their generation mechanismsare considered as complex reactions by way of such elementary reactionprocesses as shown by formulas (C36) to (C40) below.O₂ +e ⁻→O2⁻  (C36)O₂ ⁻+H⁺→.OOH  (C37)O₂+2H⁺+2e ⁻→2H₂O₂  (C38)H₂O₂+H⁺ +e ⁻→H₂O+.OH  (C39)H₂O₂→2.OH  (C40)

It is considered that generated free radicals are reduced, finally towater, by way of such elementary reaction process as shown by formulas(C41) to (C43) below, where E^(o) is a standard redox potential given interms of NHE (normal hydrogen electrode)..OOH+H⁺ +e ⁻→H₂O₂,E^(o)=1.50V  (C41)H₂O₂+2H⁺+2e ⁻→2H₂O,E^(o)=1.77V  (C42).OH+H⁺ +e ⁻→H₂O,E^(o)=2.85V  (C43)

Now controversial is hydroxy radical that has a high redox potential andhas a strong oxidizability. Hydroperoxy radical, as well as hydrogenperoxide, may take the form of hydroxy radical on the way of reductionto water. Hydroxy radical, of which redox potential is as high as 2.85V,has a short life due to the strong oxidizability and the high reactivityas radial, and reacts with any molecule else, unless it is promptlyreduced. It is considered that various deteriorations due to oxidationcontroversial in the fuel cell are mostly associated with hydroxyradical, and that the electrolyte membrane also is decomposed by hydroxyradical, as described. However, in the case the electrolyte contains acompound that has a redox cycle where it acts as a reducing agent in arange of potentials lower than the redox potential of hydroxy radicaland as an oxidizing agent in a range of potentials than the potentialwhere hydrogen peroxide acts as a reducing agent, that is, if thecompound has a redox potential within a range of 0.68V to 2.85V (NHE),this compound does act as a catalyst, decomposing hydroxy radical,hydrogen peroxide, etc.

It is known that hydrogen peroxide acts as a reducing agent on asubstance whose redox potential is higher than hydrogen peroxide, and onthe other hand, as an oxidizing agent on a substance whose redoxpotential is lower than hydrogen peroxide, as in the formula (C42) andas in a formula (C44) below.H₂O₂→O₂+2H⁺+2e ⁻,E^(o)=0.68V  (C44)

In the case a compound that has a redox potential of 0.68V or more iscontained in the electrolyte membrane, hydrogen peroxide acts as areducing agent on the compound, so that hydrogen peroxide is oxidizedand decomposed by the compound. On the other hand, in the case acompound contained in the electrolyte membrane has a redox potential of2.85V or less, this compound acts as a reducing agent on hydroxyradical, so that hydroxy radical is decomposed by reduction to water. Inthe case a fluorine system membrane is used as the electrolyte membrane,this fluorine system membrane has a tendency to be oxidized by acompound of which redox potential is 2.5V or more, and is almost fleefrom the anxiety that this electrolyte membrane may be oxidized by acompound contained in the electrolyte membrane. On the other hand, inthe case of a hydrocarbon system electrolyte used as the electrolytemembrane, substituting typical organic compounds therefor, benzene is tobe oxidized at 2.00V, toluene is at 1.93V, and xylene is at 1.58V, thusconstituting the possibility that the hydrocarbon system electrolytemembrane may be oxidized if the compound contained in the electrolytemembrane has a high redox potential. Therefore, by setting the redoxpotential of antioxidant within a range of 1.00V or less, theelectrolyte membrane can be kept from being oxidized even in use of ahydrocarbon system membrane, allowing for effective decomposition ofhydrogen peroxide and hydroxy radical.

In the solid polymer electrolyte according to this mode of embodimentthat has a compound whose redox potential ranges 0.68V to 1.00V (NHE),hydroxy radical and hydrogen peroxide are decomposed by catalysis ofthat compound included in the electrolyte membrane, without oxidizingthe electrolyte membrane. Further, the compound that has a catalysis bywhich active oxygen is decomposed takes forms of an oxidant and areductant after the decomposition of active oxygen, in a redox cycle ofthe compound, and can be used many times. It therefore is possible toimplement an electrolyte for solid high-polymer type fuel cells with amaintained duration performance.

The compound represented by the general formula (I) may comprise acompound represented by a general formula (VII) below

where R₁₃ and R₁₄ each respectively denote an alkyl group, or an alkylgroup substituted in part by an arbitrary radical, wherein R₁₃ and R₁₄may be chained, ringed, or branched R₁₃ and R₁₄ may be mutually combinedto form a ring, and may include oxygen and nitrogen atoms. In use of thecompound represented by the general formula (VII), the redox potentialis low, so that the reduction reaction of oxygen is the more promoted.In the compound represented by the general formula (VII), substituentsR₁₃ and R₁₄ may include linear chain or branch chain alkyl groups ofcarbon numbers within a range of 1 to 10 or near, for example, a methyl,an ethyl, a propyl, an isopropyl, a butyl, an isobutyl, a sec-butyl, at-butyl, a pentyl, a hexyl, a heptyl, an octyl, and a decyl group, andthe like. They may preferably have carbon numbers within a range of 1 to6 or near, or more preferably, be as lower alkyl groups as carbonnumbers within a range of 1 to 4 or near.

The compound represented by the general formula (VII) may preferablycomprise a compound represented by a general formula (VIII) below

where R₁₃ to R₁₆ each respectively denote an alkyl group, or an alkylgroup substituted in part by an arbitrary radical, wherein R₁₃ to R₁₆may be chained, ringed, or branched Among them, R₁₃ and R₁₄, or R₁₅ andR₁₆ may be mutually combined to form a ring, and they may include oxygenand nitrogen atoms. In the compound represented by the general formula(VIII), substituents R₁₃ to R₁₆ may include linear chain or branch chainalkyl groups of carbon numbers within a range of 1 to 10 or near, forexample, a methyl, an ethyl, a propyl, an isopropyl, a butyl, anisobutyl, a sec-butyl, a t-butyl, a pentyl, a hexyl, a heptyl, an octyl,and a decyl group, and the like. They may preferably have carbon numberswithin a range of 1 to 6 or near, or more preferably, be as lower alkylgroups as carbon numbers within a range of 1 to 4 or near.

The compound represented by the general formula (VIII) may preferablycomprise a compound represented by a general formula (IX) below

where a ring Y₂ denotes a 5-membered or 6-membered ring formed by R₁₃and R₁₄ mutually combined. As such rings, there may be taken, forexample, nonaromatic hydrocarbon rings, such as cycloalkene rings, e.g.,cyclohexane ring, and cycloalkene rings, e.g. cyclohexene ring,nonaromatic bridging rings, such as bridging type hydrocarbon rings,e.g. five-norbornene ring, and aromatic rings, such as benzene rings andnaphthalene rings. It is noted that those rings may have a substituentgroup.

The compound represented by the general formula (IX) may preferablycomprise a compound represented by a general formula (X) below

where Z denotes a kind of substituent selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, amino groups, andsubstituent groups including a hydrogen atom. For Z being an alkylgroup, the an alkyl group may be substituted in part by an arbitraryradical, may be chained, ringed, or branched in part, and may includeoxygen and nitrogen atoms. For Z being an aryl group, the aryl group maybe substituted in part by an arbitrary radical, and may include oxygenand nitrogen atoms. The compound represented by the general formula (X)is hardly hydrolyzable, and in use as a catalyst, it allows a long-termservice, allowing for the more reduced amount of catalyst in use.

For substituent Z in the compound represented by the general formula(X), there may be taken alkyl groups, in particular those of like alkylgroups to the before-mentioned alkyl groups, which have carbon numberswithin a range of 1 to 6 or near, while as aryl groups, there may betaken phenyl groups and naphthyl groups. There may be taken alkoxygroups, in particular those of like alkoxy groups to thebefore-mentioned alkoxy groups, which have carbon numbers within a rangeof 1 to 6 or near, and as carboxyl groups, those carboxyl groups whichhave carbon numbers within a range of 1 to 4 or near, for example. Asalkoxycarbonyl groups, there may be taken those alkoxycarbonyl groupswhose alkoxy parts have carbon numbers within a range of 1 to 10 ornear, for example, a methoxycarbonyl, an ethoxycarbonyl, a propoxycarbonyl, an isopropoxy carbonyl, a butoxycarbonyl, anisobutoxycarbonyl, a t-butoxycarbonyl, a pentyloxy carbonyl, and ahexyloxy carbonyl group, and the like. They may preferably have carbonnumbers of alkoxy parts within a range 1 to 6 or near, or morepreferably, be as lower alkoxycarbonyl groups as carbon numbers within arange of 1 to 4 or near.

As an example of compound represented by the general formula (X), thereis now taken TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). FIG. 7 showsexamples of compounds represented by the general formula (X), with TEMPOinclusive. FIG. 7 (i) shows TEMPO, which is an N-hydroxyimide derivativethat has a reversible redox-cycle, and performs oxidation and reductionby elementary reactions shown by formulas (C45) and (C46) below, whereit has a redox potential of 0.8 V.TEMPO⁺ +e ⁻→TEMPO,E^(o)=0.81V  (C45)TEMPO→TEMPO⁺ +e ⁻,E^(o)=0.81V  (C46)

The redox potential of TEMPO is higher than that of hydrogen peroxide,and lower than that of hydroxyl radical. Therefore, TEMPO acts as anoxidizing agent on hydrogen peroxide, oxidizing hydrogen peroxide tooxygen, and as a reducing agent on hydroxyl radical, reducing hydroxylradical to water.

The compound represented by the general formula (IX) may comprise acompound represented by a general formula (XI) below

where Z denotes a kind of substituent selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, ammo groups, andsubstituent groups including a hydrogen atom. For Z being an alkylgroup, the an alkyl group may be substituted in part by an arbitraryradical, may be chained, ringed, or branched in part, and may includeoxygen and nitrogen atoms. For Z being an aryl group, the aryl group maybe substituted in part by an arbitrary radical, and may include oxygenand nitrogen atoms.

The compound represented by the general formula (IX) may comprise acompound represented by a general formula (XII) below

where Z denotes a kind of substituent selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, amino groups, andsubstituent groups including a hydrogen atom. For Z being an alkylgroup, the an alkyl group may be substituted in part by an arbitraryradical, may be chained, ringed, or branched in part, and may includeoxygen and nitrogen atoms. For Z being an aryl group, the aryl group maybe substituted in part by an arbitrary radical, and may include oxygenand nitrogen atoms. Also these compounds are hardly hydrolysable likethat represented by the general formula (X), and hence, in use as acatalyst, they allow a long-term service, allowing for the more reducedamount of catalyst in use. For compounds represented by the generalformula (XI) or (XII), the substituents to be employed may be like tothe compound represented by the general formula (X).

Examples of compounds represented by the general formula (XI) or (XII)are shown in FIG. 8 to FIG. 10. As examples of compounds represented bythe general formula (XI) or (XII), PROXYL(2,2,5,5-tetraethylpyrrolidine-1-oxyl) and DOXYL(4,4-dimethyloxazolidine-3-oxyl) are now taken. Like TEMPO, thesecompounds also have their reversible redox cycles, where they have redoxpotentials within a range of 0.68V to 1.00V. Therefore, also thesecompounds act as an oxidizing agent on hydrogen peroxide, for oxidizinghydrogen peroxide to oxygen, and as a reducing on hydroxy radical, forreducing hydroxy radical to water.

FIG. 11 shows a redox cycle of TEMPO as a representative example of suchcompounds, illustrating a mechanism for inactivation of hydrogenperoxide and hydroxy radical by TEMPO. This redox mechanism is a complexreaction composed of a plurality of elementary reactions shown byformulas (C47) to (C52) below.TEMPO+O₂→TEMPO⁺+O₂ ⁻  (C47)O₂ ⁻H⁺→HOO.  (C48)TEMPO+.OOH+H⁺→TEMPO⁺+H₂O₂  (C49)TEMPO+H₂O₂+H⁺→TEMPO⁺+H₂O+.OH  (C50)TEMPO+.OH+H⁺→TEMPO⁺+H₂O  (C51)4TEMPO⁺+4e ⁻→4TEMPO  (C52)

Oxygen in the air is a triplet radical molecule in the ground state,called triplet oxygen, which cooperates with TEMPO for transfer ofelectron therebetween at a normal temperature under normal pressure, asshown by the formula (C47), to form a CT (charge transfer) complex. Atthis time, nitrogen atom of TEMPO being quaternized to N⁺, TEMPO becomesTEMPO⁺, while oxygen is activated in the form of super-oxide (O₂ ⁻).Active super-oxide immediately reacts to hydrogen ion, generating peroxyradical (.OOH). Peroxy radical has a high reduction potential of 1.50V,as a species more active than oxygen, which generates hydrogen peroxidethrough a reaction shown by the formula (C49). Though not being radical,hydrogen peroxide is yet active under presence of catalyst, and has ahigh reduction potential of 1.77V, as a species more active than oxygen,which receives electron from another TEMPO, and generates water andhydroxyl radical (.OH), as shown by the formula (C50). This TEMPO isactivated to TEMPO⁺.

Hydroxyl radical thus generated has a great reduction potential of2.85V, as a very active species, so that it receives electron fromanother TEMPO, generating water, as shown by the formula (C51). Byelementary reactions of formulas (C47) to (C51), a four-electronreduction of oxygen develops, which generates four TEMPO molecules.TEMPO⁺ also is an active species, which receives an electron (e) fromthe positive electrode of fuel cell, whereby TEMPO is recovered, asshown by the formula (C52). Like this, TEMPO has the function of afour-electron reduction of oxygen with respect to the potential, aswell, and can have a sustained function as a catalyst for vanishinghydrogen peroxide, hydroxy radical, etc. over a long term Further, TEMPOis not hydrolyzable, and can be used over a long term, thus allowing forimplementation of a solid polymer electrolyte with a maintaineddurability.

It is noted that the solid polymer electrolyte according to this mode ofembodiment can be employed as an electrolyte membrane for fell cells ofa solid high-polymer type. In this respect, for fuel cells using ahigh-polymer electrolyte membrane of proton exchange type, no limitationis provided to the kind of fuel, and it is employable to any one of ahydrogen type fuel cell, a direct methanol type fuel cell, a directhydrocarbon type fuel cell, and the like. This case allows forimplementation of a fuel cell with a maintained durability.

This fuel cell is employable for fuel cell systems using a high-polymerelectrolyte membrane of proton exchange type, and has applicationsthereof not limited to a fuel cell vehicle, and is applicable, forexample, to a fuel cell cogeneration power generating system, a fuelcell home electric appliance, a fuel cell portable device, a fuel celltransport machine, and the like.

EXAMPLES

Description will be made of fuel cell systems according to examples 1 to12 of embodiment of the invention and comparative examples 1 and 2,while the scope of the invention is not limited thereto. Those examplesexemplify fuel cell systems using different antioxidants, examiningtheir effectiveness.

<Sample Preparation>

Example 1

A film of Nafion® 117 of Du Pont Co., 175 μm thick, was cut into 1 cmsquares to be used as solid polymer electrolyte membranes. Nafion®membranes were pretreated to the NEDO PEFC R&D project standardtreatment, where they were boiled: in 3% hydrogen peroxide aqueoussolution for 1 hour, and in distilled water for 1 hour, then, in 1Msulfuric acid solution for 1 hour, and finally, in distilled water for 1hour, in this order.

Next, for a facilitated ageing resistance judgment in endurance test,pretreated Nafion® membranes were subjected to an ion exchangetreatment, where they were soaked in 100 mM FeSO₄ aqueous solution forone night or more, and ultrasonically cleaned in distilled water for 15minutes, for removing ions adhering to membrane to thereby exchangecounter ions of Nafion® from H⁺ to Fe2⁺. Wako pure medicine high gradeFeSO₄.7H₂O was used as a reagent Next, platinum-supporting carbon (20 wt% Pt/Vulcan XC-72, Cabot Co.) was coated 1 mg/cm² on both sides of eachion-exchanged electrolyte membrane to fabricate a membrane electrodeassembly (MEA). Fabricated MBA was assembled in a single cell, toprovide a unit cell for PEFC to be 5 cm². 70° C. humidified hydrogen gas(atmospheric pressure) as the fuel electrode side gas and 70° C.humidified oxygen gas (atmospheric pressure) as the air electrode sidegas were supplied, via bubblers, to a unit cell held 70° C. As anantioxidant for inactivation of active oxygen, 1 mM NHPI aqueoussolution was fed, using a liquid feed pump, by a flow rate of 1cm³/minute to a fuel gas port. The unit cell was controlled to be held70° C.

Example 2

Instead of NHPI aqueous solution, NHMI (N-hydroxy maleic acid imide)aqueous solution was used as an antioxidant for example 2, of whichtreatment was like to example 1.

Example 3

Instead of NHPI aqueous solution, NHSI (N-hydro-oxy succinic acid imide)aqueous solution was used as an antioxidant for example 3, of whichtreatment was like to example 1.

Example 4

Instead of NHPI aqueous solution, NHGI (N-hydroxyglutaric acid imide)aqueous solution was used as an antioxidant for example 4, of whichtreatment was like to example 1.

Example 5

Instead of NHPI aqueous solution, THICA (N,N′,N″-trihydroxy isocyanuricacid) aqueous solution was used as an antioxidant for example 5, ofwhich treatment was like to example 1.

Comparative Example 1

Comparative example 1 was set to the example 1, as it had no antioxidantaqueous solution fed.

In example 6 to example 11, an S-PES (sulfonated polyethersulfone) filmwas employed as solid polymer electrolyte membrane. As the S-PES filmprocured and used was an equivalent to that described in p. 31 of“researches and developments of a durability-elevated hydrocarbon systemelectrolyte membrane for proton-exchange membrane fuel cells in theproton-exchange membrane fuel cell elements technology development andlike program in the proton-exchange membrane fuel cell system technologyproject”, 2002 yearly results report of the New Energy and IndustrialTechnology Development Organization of Japan.

Example 6

An S-PES film 170 μm thick was cut into 1 cm squares, andplatinum-supporting carbon (Cabot Co. make 20 wt % Pt/Vulcan XC-72) wascoated by 1 mg/cm² on both sides of an S-PES membrane to fabricate amembrane electrode assembly (MEA). Fabricated MEA was assembled in asingle cell, to provide a unit cell for PEFC, which was a 5-cm² unitcell. 70° C. humidified hydrogen gas (atmospheric pressure) as the fuelelectrode side gas and 70° C. humidified oxygen gas (atmosphericpressure) as the air electrode side gas were supplied, via bubblers, toa unit cell held 70° C. As an antioxidant for inactivation of activeoxygen, 1 mM TEMPO-OH aqueous solution was fed, using a liquid feedpump, by a flow rate of 1 cm³/minute to a fuel gas port. The unit cellwas controlled to be held 70° C.

Example 7

Instead of TEMPO-OH aqueous solution, TEMPO-COOH (Aldrich Co.) aqueoussolution was used as an antioxidant for example 7, of which Eminent waslike to example 6.

Example 8

Instead of TEMPO-OH aqueous solution, TEMPO (Aldrich Co.) aqueoussolution was used as an antioxidant for example 8, of which treatmentwas like to example 6.

Example 9

Instead of TEMPO-OH aqueous solution, PROXYL-CONH₂ (Aldrich Co.) aqueoussolution was used as an antioxidant for example 9, of which treatmentwas like to example 6.

Example 10

Instead of TEMPO-OH aqueous solution, PROXYL-COOH (Aldrich Co.) aqueoussolution was used as an antioxidant for example 10, of which treatmentwas like to example 6.

Example 11

Instead of TEMPO-OH aqueous solution,3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-yloxy (Aldrich Co.) aqueoussolution was used as an antioxidant for example 11, of which treatmentwas like to example 6.

Example 12

Instead of TEMPO-OH aqueous solution, DTBN (di-t-butylnitroxide: AldrichCo.) aqueous solution was used as an antioxidant for example 12, ofwhich treatment was like to example 6.

Comparative Example 2

Comparative example 2 was set to the example 6, as it had no antioxidantaqueous solution fed.

Samples of the foregoing examples were evaluated, as follows:

<Measurements of Redox Potential>

Redox potentials of the compounds employed in the examples were measuredby using glassy carbon as an acting electrode, platinum as a counterelectrode, a saturated calomel electrode (SCE) as a reference electrode,and 1M sulfuric acid as an electrolytic solution. Exemplary measurementsof NHPI as a typical imide system compound and TEMPO as a typical TEMPOsystem compound are shown in FIG. 14 and FIG. 15. Graphs in FIG. 14 andFIG. 15 are corrected to standard potential E^(o) (NHE) to be consistentwith redox potentials of respective substances. Redox potential of NHPIresides near 1.10V (SCE) as shown in FIG. 14, and redox potential ofTEMPO resides near 0.57V (SCE) as seen from FIG. 15. As will be seenfrom those potentials, NHPI and TEMPO are compounds that function as areducing agent on hydroxy radical, and compounds that function as anoxidizing agent on hydrogen peroxide, as compounds meeting the presentobjective.

<Start and Stop Repeating Endurance Test>

An open-circuit condition was held at the fuel electrode side for 30minutes, to start the test. In the test, supplying gas to the unit cellby a flow rate of 300 dm³/min, the current density was increased from adischarge open-circuit condition, making discharge till the terminalvoltage gets below 0.3V. Then, after the terminal voltage had got below0.3V, an open-circuit condition was again held for 5 minutes. Thisoperation was repeated for comparison of the endurance performance interms of the number of times when the voltage gets below 0.4V under acondition of power generation with a current density of 1 mA/cm³. FIG.16 shows, as an example of a start and stop repeating endurance test fora unit cell of fuel cell prepared in the example 1, a graph of initialvalues of current-voltage curve, and a current-voltage curve afterduration. In this graph, under the condition of power generation withthe current density of 1 mA/cm³, the voltage gets below 0.4V at a numberof times, which is referred to as start-stop repetition time number.

<Analysis of Emitted Substances at Air Electrode>

For analysis of deterioration of Nafion® membrane, measurements weremade of concentrations of fluoride ions and sulfate ions emitted upondecomposition of the membrane. For S-PES membrane, concentrationmeasurements were made of sulfate ions emitted upon membranedecomposition. For detection of transferred ions, discharged liquid fromthe air electrode was collected, and measured by using an ionchromatograph The ion chromatograph was a Daionecc Co. make (model name:CX-120). As a specific test method for respective examples, as well asfor comparative examples, comparison was made of samples of liquiddischarged from the air electrode upon a completion of 100 times ofrepetition in the start and stop repeating endurance test. Further,gases emitted at the air electrode were measured by using a gaschromatograph mass spectrometer. The gas chromatograph mass spectrometerused was a Shimadzu Co. make (GCMS-QP5050).

For example 1 to example 12 and comparative example 1 and comparativeexample 2, the type of electrolyte membrane, used antioxidant, redoxpotential of antioxidant, start-stop repetition time number, andpresence or absence of emission of fluoride ion, sulfate ion, and carbondioxide at the air electrode are listed in Table 1 and Table 2 below.TABLE 1 Electrolyte Anti- Redox Repetition Fluoride Sulfate Membraneoxidant Pot.*) Time No. Ion**) Ion**) CO₂**) Example 1 Nafion NHPI 1.34V 1250 No No Yes Example 2 Nafion NHMI 1.34 V 1170 No No Yes Example 3Nafion NHSI 1.36 V 1210 No No Yes Example 4 Nafion NHGI 1.38 V 1430 NoNo Yes Example 5 Nafion THICA 1.40 V 1450 No No Yes Com. Ex 1 NafionNone — 120 Yes Yes No*)Redox potential of antioxidant**)Yes: Present, No: Absent

TABLE 2 Electrolyte Anti- Redox Repetition Fluoride Sulfate Membraneoxidant Pot.*) Time No. Ion**) Ion**) CO₂**) Example 6 S-PES TEMPO-OH0.81 V 740 — No Yes Example 7 S-PES TEMPO-COOH 0.81 V 760 — No YesExample 8 S-PES TEMPO 0.81 V 730 — No Yes Example 9 S-PES PROXYL-CONH₂0.85 V 730 — No Yes Example 10 S-PES PROXYL-COOH 0.86 V 730 — No YesExample 11 S-PES ***) 0.95 V 610 — No Yes Example 12 S-PES DTBN 0.80 V650 — No Yes Com. Ex 2 S-PES None — 80 — Yes No*)Redox potential of antioxidant**)Yes: Present, No: Absent***)3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-yloxy

The antioxidants employed in example 1 to example 12 had their redoxpotentials within a range of a potential of 0.68V (NHE) where hydrogenperoxide acts as an oxidizing agent and a potential of 1.77V (NHE) wherehydrogen peroxide acts as a reducing agent, thus meeting the objective.

For the comparative example 1 supplied with no antioxidant, the startand stop repeating endurance test showed, under the condition of powergeneration with a current density of 1 mA/cm³, a voltage drop to a levelof 0.4 V or less at a start-stop repetition time number of 120. To thecontrary, in each of example 1 to example 5 supplied with anantioxidant, the voltage dropped below 0.4V about a start-stoprepetition time number of 1200, as a verification of a suppresseddeterioration of solid polymer electrolyte membrane by addition of theantioxidant, with an enhanced durability. In example 6 to example 12,the voltage dropped below 0.4V at a start-stop repetition time number of600 or more, which verified an enhanced endurance due to suppresseddeterioration of electrolyte membrane.

Ion chromatograph analyses revealed a detection of fluoride ion andsulfate ion in the comparative example 1, and a detection of sulfate ionin the comparative example 2, supporting a deterioration bydecomposition of electrolyte membrane. On the contrary, in each ofexample 1 to example 5, emission of fluoride ion and sulfate ion wasbelow detection limits, proving a suppressed decomposition of Nafion®membrane by introduction of antioxidant In each of example 6 to example12, as well, emission of sulfate ion was below detection limits, provinga suppressed decomposition of S-PES membrane by introduction ofantioxidant For example 1 to example 12 where an antioxidant wasintroduced, measurements by gas chromatograph mass spectrometer revealeda detection of CO₂, supporting that the antioxidant, as it had beenintroduced from the fuel electrode and served for inactivation of activeoxygen, was oxidized at the air electrode, emitting CO₂.

Although perfluorosulfonic acid system polymers typified by the Nafion®film having wide application to an electrolyte membrane of a fuel cellin a fuel cell system, as well as hydrocarbon system polymers referredto S-PES, are put in a situation where they are unsuccessfullyconsidered having a sufficient tolerance by generation of active oxygenat an air electrode of the fuel cell, as will be seen from the foregoingdescription, by mixing or dissolving an antioxidant to a liquid fuelaccording to an embodiment of the invention, active oxygen can bedecomposed even if the generation is continuous, thus enabling aprevention of deterioration of the electrolyte membrane, allowing for anenhanced durability of fuel cell system.

Description is now specifically made of solid polymer electrolytesaccording to examples 13 to 34 of embodiment of the invention andcomparative examples 3 and 4, while the scope of the invention is notlimited thereto. Those examples exemplify solid polymer electrolytesprepared with different materials, examining their effectiveness.

sample Preparation>

Example 13

A film of Nafion® 117 of Du Pont Co., 175 μm thick, was cut into 1 cmsquares to be used. Nafion® membranes were pretreated to the NEDO PEFCR&D project standard treatment, where they were boiled: in 3% hydrogenperoxide aqueous solution for 1 hour, and in distilled water for 1 hour,then, in 1M sulfuric acid solution for 1 hour, and finally, in distilledwater for 1 hour, in this order.

Next, as a catalyst for vanishment of active oxygen, i.e., as an activeoxygen washing catalyst, 0.5 mM of NHPI was added to pretreated Nafion®membranes, which were thereafter soaked, for evaluation use, for 12hours at 80° C. in 10 cm³ of 10% hydrogen peroxide, as it was preparedby diluting 30% hydrogen peroxide solution (Wako pure medicine highgrade) with ultra-pure water.

Example 14

For promoting deterioration in hydrogen peroxide endurance test,pretreated Nafion® membranes were soaked in 100 mM FeSO₄ aqueoussolution for one night or more, and ultrasonically cleaned in distilledwater for 15 minutes, for removing ions adhering to membrane to therebyexchange counter ions of Nafion® from H⁺ to Fe²⁺. Wako pure medicinehigh grade FeSO₄.7H₂O was used as a reagent.

Next, like the example 13, 0.5 mM of NHPI was added as an active oxygenwashing catalyst to ion-exchanged Nafion® membranes, which werethereafter soaked, for evaluation use, for 12 hours at 80° C. in 10 cm³of 10% hydrogen peroxide, as it was prepared by diluting 30% hydrogenperoxide solution (Wako pure medicine high grade) with ultra-pure water.

Example 15

Instead of NHPI aqueous solution, 0.5 mM of N-hydroxymaleic acid imideaqueous solution was used as an active oxygen vanishing catalyst forexample 15, of which treatment was like to example 13.

Example 16

Instead of NHPI aqueous solution, 0.5 mM of N-hydroxymaleic acid imideaqueous solution was used as an active oxygen vanishing catalyst forexample 16, of which treatment was like to example 14.

Example 17

Instead of NHPI aqueous solution, 0.5 mM of N-hydroxysuccinic acid imideaqueous solution was used as an active oxygen vanishing catalyst forexample 17, of which treatment was like to example 13.

Example 18

Instead of NHPI aqueous solution, 0.5 mM of N-hydroxymaleic acid imideaqueous solution was used as an active oxygen vanishing catalyst forexample 18, of which treatment was like to example 14

Example 19

Instead of NHPI aqueous solution, 0.5 mM of N-hydroxytrimellitic acidimide aqueous solution was used as an active oxygen vanishing catalystfor example 19, of which treatment was like to example 13.

Example 20

Instead of NHPI aqueous solution, 0.5 mM of N-hydroxytrimellitic acidimide aqueous solution was used as an active oxygen vanishing catalystfor example 20, of which treatment was like to example 14.

Example 21

Instead of NHPI aqueous solution, 0.5 mM of N,N′-dihydroxypyromelliticacid imide aqueous solution was used as an active oxygen vanishingcatalyst for example 21, of which treatment was like to example 13.

Example 22

Instead of NHPI aqueous solution, 0.5 mM of N,N′-dihydroxypyromelliticacid imide aqueous solution was used as an active oxygen vanishingcatalyst for example 22, of which treatment was like to example 14.

Example 23

Instead of NHPI aqueous solution, 0.5 mM of N-hydroxyglutaric acid imide(NHGI) aqueous solution was used as an active oxygen vanishing catalystfor example 23, of which treatment was like to example 13.

Example 24

Instead of NHPI aqueous solution, 0.5 mM of N-hydroxyglutaric acid imide(NHGI) aqueous solution was used as an active oxygen vanishing catalystfor example 24, of which treatment was like to example 14.

Example 25

Instead of NHPI aqueous solution, 0.5 mM ofN-hydroxy-1,8-naphthalenedicarboxylic acid imide (NHNDI) aqueoussolution was used as an active oxygen vanishing catalyst for example 25,of which treatment was like to example 13.

Example 26

Instead of NHPI aqueous solution, 0.5 mM ofN-hydroxy-1,8-naphthalenedicarboxylic acid imide (NHNDI) aqueoussolution was used as an active oxygen vanishing catalyst for example 26,of which treatment was like to example 14.

Example 27

Instead of NHPI aqueous solution, 0.5 mM ofN-hydroxy-1,8-decalindicarboxylic acid imide (NHDDI) aqueous solutionwas used as an active oxygen vanishing catalyst for example 27, of whichtreatment was like to example 13.

Example 28

Instead of NHPI aqueous solution, 0.5 mM ofN-hydroxy-1,8-decalindicarboxylic acid imide (NHDDI) aqueous solutionwas used as an active oxygen vanishing catalyst for example 28, of whichtreatment was like to example 14.

Example 29

Instead of NHPI aqueous solution, 0.5 mM ofN,N′-dihydroxy-1,8,4,5-naphthalenetetracarboxylic acid imide (NHNTI)aqueous solution was used as an active oxygen vanishing catalyst forexample 29, of which treatment was like to example 13.

Example 30

Instead of NHPI aqueous solution, 0.5 mM ofN,N′-dihydroxy-1,8,4,5-naphthalenetetracarboxylic acid imide (NHNTI)aqueous solution was used as an active oxygen vanishing catalyst forexample 30, of which treatment was like to example 14.

Example 31

Instead of NHPI aqueous solution, 0.5 mM ofN,N′-dihydroxy-1,8,4,5-decalintetracarboxylic acid imide (NHDTI) aqueoussolution was used as an active oxygen vanishing catalyst for example 31,of which treatment was like to example 13.

Example 32

Instead of NHPI aqueous solution, 0.5 mM ofN,N′-dihydroxy-1,8,4,5-decalintetracarboxylic acid imide (NHDTI) aqueoussolution was used as an active oxygen vanishing catalyst for example 32,of which treatment was like to example 14.

Example 33

Instead of NHPI aqueous solution, 0.5 mM ofN,N′,N′-trihydroxyisocyanuric acid (THICA) aqueous solution was used asan active oxygen vanishing catalyst for example 33, of which treatmentwas like to example 13.

Example 34

stead of NHPI aqueous solution, 0.5 mM of N,N′,N″-trihydroxyisocyanuricacid (THICA) aqueous solution was used as an active oxygen vanishingcatalyst for example 34, of which treatment was like to example 13.

Comparative Example 3

For comparative example 3, Nafion® membranes were pretreated by likemethod to example 13, without addition of active oxygen vanishingcatalyst.

Comparative Example 4

For comparative example 4, Nafion® membranes were ion-exchanged by likemethod to example 14, without addition of active oxygen vanishingcatalyst.

Nafion® membranes processed in the foregoing methods were evaluated inthe following methods.

<Hydrogen Peroxide Endurance Test>

Nafion® membranes processed in the described manner were observed tovisually check for changes in color.

<Deterioration Analysis of Membrane>

For deterioration analysis of membrane, measurements were made ofconcentrations of fluoride ions generated upon decomposition of Nafion®membrane. For detection of fluoride ions, a solution of sample preparedin the described manner was diluted ten-times with ultrapure water, andthis diluted solution was measured by an ion chromatograph. The ionchromatograph used was a Daionecc Co. make model: DX-AQ).

<Stability Test of Active Oxygen Vanishing Catalyst>

For stability test of NHPI and NHGI as active oxygen vanishingcatalysts, an NHPI solution and an NHGI solution, prepared to in M, wereeach respectively put in a beaker, held at 80° C., and sampled every 24hours, to be liquid-chromatographed. For NHPI and NHGI, theconcentration was measured by a peak area of chromatogram thus obtained.

For example 13 to example 34 and comparative example 3 and comparativeexample 4, ion types of Nafion® membrane, kinds of catalyst, fluorideion concentrations, and color changes of membrane are listed in Table 3and Table 4 below. TABLE 3 Active oxygen Nafion vanishing Fluoride ionColor changes membrane catalysts concentrations of membrane Example 13H⁺ type NHPI 0.1 ppm or less None Example 14 Fe²⁺ type NHPI 0.2 ppm orless None Example 15 H⁺ type N-hydroxymaleic 0.1 ppm or less None acidimide Example 16 Fe²⁺ type N-hydroxymaleic 0.2 ppm or less None acidimide Example 17 H⁺ type N-hydroxysuccinic 0.1 ppm or less None acidimide Example 18 Fe²⁺ type N-hydroxysuccinic 0.2 ppm or less None acidimide Example 19 H⁺ type N-hydroxytrimellitic 0.1 ppm or less None acidimide Example 20 Fe²⁺ type N-hydroxytrimellitic 0.2 ppm or less Noneacid imide Example 21 H⁺ type N,N′-dihydroxypyromellitic 0.1 ppm or lessNone acid imide Example 22 Fe²⁺ type N,N′-dihydroxypyromellitic 0.3 ppmNone acid imide Comparative H⁺ type None 0.1 ppm or less None Example 3Comparative Fe²⁺ type None 2.0 ppm Tanned Example 4

TABLE 4 Active oxygen Color Nafion vanishing Fluoride ion changesmembrane catalysts concentrations of membrane Example 23 H⁺ type NHGI0.1 ppm or less None Example 24 Fe²⁺ type NHGI 0.2 ppm or less NoneExample 25 H⁺ type NHNDI 0.1 ppm or less None Example 26 Fe²⁺ type NHNDI0.2 ppm or less None Example 27 H⁺ type NHDDI 0.1 ppm or less NoneExample 28 Fe²⁺ type NHDDI 0.2 ppm or less None Example 29 H⁺ type NHNTI0.1 ppm or less None Example 30 Fe²⁺ type NHNTI 0.2 ppm or less NoneExample 31 H⁺ type NHDTI 0.1 ppm or less None Example 32 Fe²⁺ type NHDTI0.3 ppm None Example 33 H⁺ type THICA 0.1 ppm or less None Example 34Fe²⁺ type THICA 0.3 ppm None

As will be seen from results of examples 13, 15, 17, 19, 21, 23, 25, 27,29, 31, and 33, and comparative example 3, for Nafion® membranes of H⁺type, fluoride ions were hardly detected, irrespective of presence orabsence of an active oxygen vanishing catalyst such as NHPI or NHGI,even after 12 hours of treatment in hydrogen peroxide solution,supporting no progress of deterioration in Nafion® membrane. To thecorny, as shown by comparative example 4, for Nafion® membranes of Fe²⁺type, fluoride ions were detected after 12-hour treatment, supporting aprogress of deterioration in membrane.

Further, as will be seen from results of examples 14, 16, 18, 20, 22,24, 26, 28, 30, 32, and 34, and comparative example 4, detected fluorideions had a difference often times or more depending on presence orabsence of an active oxygen vanishing catalyst such as NHPI or NHGI,proving a suppressed decomposition of Nafion® membrane under presence ofan active oxygen vanishing catalyst such as NHPI or NHGI.

Further, for Nafion® membrane of Fe²⁺ type which is almost transparent,comparative example 4 showed a change to a tannish color of membraneafter 12-hour treatment in hydrogen peroxide solution, as an oxidationof counter ion from Fe²⁺ to Fe³⁺, suggesting an occurrence of Fentonreaction. To the contrary, in examples 14, 16, 18, 20, 22, 24, 26, 28,30, 32, and 34 where an active oxygen vanishing catalyst such as NHPI orNHGI was present, no change of color was observed despite a presence ofFe²⁺, proving a suppression of Fenton reaction.

Further, during 12 hours of heating treatment in hydrogen peroxidesolution, sample bottles of examples 13 and 14, where NHPI had been put,generated more bubbles than comparative examples 3 and 4, which wasobserved as a visual check of promoted decomposition of hydrogenperoxide. It is noted that results of this time showed no greatdifferences in amounts of fluoride ions generated in dependence on thekind of active oxygen vanishing catalyst.

Next, using membranes obtained from examples 14, 16, 18, 20, 22, 24, 26,28, 30, 32, and 34, and comparative example 4 as electrolyte membranes,and platinum-supporting carbon as electrodes, unit cells for fuel cellwere prepared, and subjected to a start and stop repeating endurancetest. On electrolyte membrane obtained, platinum-supporting carbon (20wt % Pt/Vulcan XC-72, Cabot Co.) was coated by a spread of 1 mg/cm² tothe sides to be anode and cathode, to fabricate a membrane electrodeassembly (MEA). Fabricated MEA was assembled in a single cell, toprovide a unit cell for PEFC to be used for evaluation. The unit cellwas a 5 cm² simplex cell.

The start and stop repeating endurance test was made in the followingmanner.

<Start and Stop Repeating Endurance Test>

70° C. humidified hydrogen gas (atmospheric pressure) as an anode gasand 70° C. humidified oxygen gas (atmospheric pressure) as a cathode gaswere supplied to a unit cell held 70° C., and an open circuit conditionwas held for 30 minutes, to start the test.

In the test, supplying gas to the unit cell by a flow rate of 300dm³/min, the current density was increased from a discharge open-circuitcondition, making discharge till the terminal voltage gets below 0.3V.Then, after the terminal voltage had got below 0.3V, an open-circuitcondition was again held for 5 minutes. This operation was repeated, forcomparison of the endurance performance in terms of the number of timeswhen the voltage gets below 0.4V under a condition of power generationwith a current density of 1 mA/cm³. For promotion of the endurance test,Nafion® membranes employed were taken from examples 14, 16, 18, 20, 22,24, 26, 28, 30, 32, and 34 and comparative example 4, where they wereeach replaced by Fe²⁺ type. For examples 14, 16, 18, 20, and 22 andcomparative example 4, the counter ion types of Nafion® membrane, kindsof catalyst, and start-stop repetition time number are listed in Table 5below. TABLE 5 start-stop Nafion Active oxygen repetition membranesvanishing catalysts time number Example 14 Fe²⁺ type NHPI 550 Example 16Fe²⁺ type N-hydroxymaleic 510 acid imide Example 18 Fe²⁺ typeN-hydroxysuccinic 580 acid imide Example 20 Fe²⁺ typeN-hydroxytrimellitic 530 acid imide Example 22 Fe²⁺ type N,N′- 550dihydroxypyromellitic acid imide Comparative Fe²⁺ type None 70 example 4

As shown in comparative example 4, absence of active oxygen vanishingcatalyst resulted, under the condition of power generation with acurrent density of 1 mA/cm³, in a voltage drop to a level of 0.4 V orless at a start-stop repetition time number of 70. To the contrary, ineach case in which of an active oxygen vanishing catalyst was added, thevoltage dropped below 0.4V at a start-stop repetition time number over500, as a verification of a suppressed deterioration of electrolytemembrane due to addition of active oxygen vanishing catalyst theantioxidant Further, in any of examples 24, 26, 28, 30, 32, and 34 wherethe active oxygen vanishing catalyst was a six-membered ring type, thevoltage dropped below 0.4V at a start-stop repetition time number over600, as a support for an increased tendency to suppress deterioration ofelectrolyte membrane, with an excellent durability, in comparison withexamples 14, 16, 18, 20, and 22 of a five-membered ring type. This isconsidered because imide compounds of six-membered ring type have abetter and more stable resistance to hydrolysis than five-membered ringtype, so that they have an increased start-stop repetition time numberin comparison with imide compounds of five-membered ring type. FIG. 17shows, in a graph, results of stability test on NHPI and NHGI. As shownin FIG. 7, NHPI, which is a five-membered ring type, had a decreasingconcentration by hydrolysis shown in formula (C23) as the time hadelapsed, so that the concentration of NHPI was decreased to approx. 0.6mM by a lapse of 96 hours. To the contrary, although NHGI of asix-membered ring type also had a decreasing concentration by hydrolysisshown in formula (C24) as the time had elapsed, the concentration after96 hours was yet approx-0.9 mM.

It is understood that although imide compounds of six-membered ring typealso have hydrolysis progress like five-membered ring type, thehydrolysis reaction is slower in comparison with five-membered ringtype, so that the resistance to hydrolysis is good and stable. Amongothers, in the case of example 34 that employed THICA as an activeoxygen vanishing catalyst, the voltage dropped below 0.4V at astart-stop repetition time number of 850, as a verification of aspecifically excellent durability. It is considered that THICA is smallin molecular weight and has three intramolecular hydroxyimide groups,whereby it has an increased excellence in hydrolysis for vanishment ofactive oxygen, allowing for favorable durability.

Although perfluorosulfonic acid system polymers typified by the Nafion®film having wide application to the electrolyte membrane are put in asituation where they are unsuccessfully considered having a sufficienttolerance by generation of active oxygen at the positive electrode offuel cell, as will be seen from the foregoing description, by use of anelectrolyte membrane containing the above-noted compound as a catalyst,entry and exit of hydrogen radical can be repeated and they can have amaintained effect to vanish active oxygen over a long term. Like this,by use of active oxygen vanishing catalyst, even if hydrogen peroxide isgenerated at the positive electrode of solid high-polymer type fuelcell, the electrolyte membrane can have a maintained performance over along term, allowing a suppressed deterioration of electrolyte membrane,allowing for an enhanced durability of fuel cell.

Description is now specifically made of solid polymer electrolytesaccording to examples 35 to 54 of embodiment of the invention andcomparative examples 5 and 6.

<Sample Preparation>

For example 35 to example 44 and comparative example 5, an S-PES(sulfonated polyethersulfone) film was employed as a hydrogen carbonsystem electrolyte membrane. As the S-PES film, procured and used was anequivalent to that described in p. 31 of “researches and developments ofa durability-elevated hydrocarbon system electrolyte membrane forproton-exchange membrane fuel cells in the proton-exchange membrane fuelcell elements technology development and like program in theproton-exchange membrane fuel cell system technology project”, 2002yearly results report of the New Energy and Industrial TechnologyDevelopment Organization of Japan.

Example 35

An S-PES film 150 μm thick was cut into 1 cm squares to be used. Forpretreatment, S-PES membranes were boiled in distilled water for 1 hour,then in 1M sulfuric acid solution for 1 hour, and finally, in distilledwater for 1 hour, in this order. Next, as a compound for decomposingactive oxygen, 4-hydroxy-TEMPO (Aldrich Co.) shown in FIG. 7 (ii) wasadded to be 0.5 mM to p S-PES membranes, which were thereafter soaked,for evaluation use, for 24 hours at 80° C. in 10 cm³ of 0.5% hydrogenperoxide, as it was prepared by diluting 30% hydrogen peroxide solution(Wako pure medicine high grade) with ultrapure water.

Example 36

As a compound for decomposition of active oxygen, instead of4-hydroxy-TEMPO, 0.5 mM of 4-carboxy-TEMPO (Aldrich Co.) aqueoussolution was added for example 36. Other treatments were like to example35.

Example 37

As a compound for decomposition of active oxygen, 0.5 mM of TEMPO(Aldrich Co.) aqueous solution was added for example 37. Othertreatments were like to example 35.

Example 38

As a compound for decomposition of active oxygen, 0.5 mM of3-carbamoyl-PROXYL (Aldrich Co.) aqueous solution was added for example38. Other treatments were like to example 35.

Example 39

As a compound for decomposition of active oxygen, 0.5 mM of3-carboxy-PROXYL (Aldrich Co.) aqueous solution was added for example39. Other treatments were like to example 35.

Example 40

As a compound for decomposition of active oxygen, 0.5 mM of3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-yloxy (Aldrich Co.) aqueoussolution was added for example 40. Other treatments were like to example35.

Example 41

As a compound for decomposition of active oxygen, 0.5 mM ofdi-t-butylnitroxide (Aldrich Co.) aqueous solution was added for example41. Other treatments were like to example 35.

Example 42

As a compound for decomposition of active oxygen, 0.5 mM of N-hydroxyphthalic acid imide (NHPI: Aldrich Co.) aqueous solution was added forexample 42. Other treatments were like to example 35.

Example 43

As a compound for decomposition of active oxygen, 0.5 mM of N-hydroxymaleic acid imide (NHMI: Aldrich Co.) aqueous solution was added forexample 43. Other treatments were like to example 35.

Example 44

As a compound for decomposition of active oxygen, 0.5 mM of N-hydroxysuccinic acid imide (NHSI: Aldrich Co.) aqueous solution was added forexample 44. Other treatments were like to example 35.

Comparative Example 5

For comparative example 5, no compound for decomposition of activeoxygen was added to S-PES membranes pretreated by like method to example36. Other treatments were like to example 35.

For example 45 to example 54 and comparative example 6, a film ofNafion® 117 of Du Pont Co., 175 lam thick, was cut into 1 cm squares tobe used as fluorine system electrolyte membranes. Nafion® membranes wereprated to the NEDO PEFC R&D project standard treatment, where they wereboiled: in 3% hydrogen peroxide aqueous solution for 1 hour, and indistilled water for 1 hour, then, in 1M sulfuric acid solution for 1hour, and finally, in distilled water for 1 hour, in this order.Fluorine system electrolyte membranes are strong in duration relative tohydrocarbon system membranes, and for promoting deterioration inhydrogen peroxide endurance test pretreated Nafion® membranes weresoaked in 100 mM FeSO₄ aqueous solution for one night or more, andultrasonically cleaned in distilled water for 15 minutes, for removingions adhering to membrane to thereby exchange counter ions of Nafion®from 14+ to Fe²⁺. Wako pure medicine high grade FeSO₄.7H₂O was used as areagent.

As a compound for decomposition of active oxygen, 0.5 mM of4-hydroxy-TEMPO (Aldrich Co.) aqueous solution was added toion-exchanged Nafion® membranes, which were thereafter soaked, forevaluation use, for 12 hours at 80° C. in 10 cm³ of 10% hydrogenperoxide, as it was prepared by diluting 30% hydrogen peroxide solution(Wako pure medicine high grade) with ultrapure water.

Example 46

Instead of 4-hydroxy-TEMPO, 0.5 mM of 4-carboxy-TEMPO (Aldrich Co.)aqueous solution was added for example 46. Other treatments were like toexample 45.

Example 47

As a compound for decomposition of active oxygen, 0.5 mM of TEMPO(Aldrich Co.) aqueous solution was added for example 47. Othertreatments were like to example 45.

Example 48

As a compound for decomposition of active oxygen, 0.5 mM of3-carbamoyl-PROXYL (Aldrich Co.) aqueous solution was added for example48. Other treatments were like to example 45.

Example 49

As a compound for decomposition of active oxygen, 0.5 mM of3-carboxy-PROXYL (Aldrich Co.) aqueous solution was added for example49. Other treatments were like to example 45.

Example 50

As a compound for decomposition of active oxygen, 0.5 mM of3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-yloxy (Aldrich Co.) aqueoussolution was added for example 50. Other treatments were like to example45.

Example 51

As a compound for decomposition of active oxygen, 0.5 mM ofdi-t-butylnitroxide (Aldrich Co.) aqueous solution was added for example51. Other treatments were like to example 45.

Example 52

As a compound for decomposition of active oxygen, 0.5 mM of NHPI(Aldrich Co.) aqueous solution was added for example 52. Othertreatments were like to example 45.

Example 53

As a compound for decomposition of active oxygen, 0.5 mM of NHMI(Aldrich Co.) aqueous solution was added for example 53. Othertreatments were like to example 45.

Example 54

As a compound for decomposition of active oxygen, 0.5 mM of NHSI(Aldrich Co.) aqueous solution was added for example 54. Othertreatments were like to example 45.

Comparative Example 6

For comparative example 6, no compound for decomposition of activeoxygen was added in example 45. Other treatments were like to example45.

S-PES membranes and Nafion® membranes treated in the foregoing methodswere evaluated in the following methods.

<Deterioration Analysis of Membrane>

For deterioration analysis of membranes, measurements were made forS-PES membranes, of concentrations of sulfate ions generated upondecomposition, and for Nafion® membranes, concentrations of fluorideions generated upon decomposition of membrane. For detection of elutedions, a solution of sample prepared in the described manner was dilutedten-times with ultrapure water, and this diluted solution was measuredby an ion chromatograph. The ion chromatograph used was a Daionecc Co.make (Model: DX-AQ).

<Measurements of Redox Potential>

Redox potentials of the compounds employed in the examples were measuredby using glassy carbon as an acting electrode, platinum as a counterelectrode, a orated calomel electrode (SCE) as a reference electrode,and 1M sulfuric acid as an electrolytic solution. Corrections are madeto standard potential E^(o) (NHE) to be consistent with redox potentialsof respective substances.

For examples 35 to 44 using S-PES membranes as electrolyte membranes,and comparative example 5, employed compounds for decomposition ofactive oxygen, redox potentials of compounds, and concentrations ofsulfite ion detected after hydrogen peroxide endurance are listed inTable 6. TABLE 6 Concen- trations Compounds for Redox of Electrolytedecomposition potentials of sulfate membranes of active oxygen compoundsion Example 35 S-PES TEMPO-OH 0.81 0.7 membrane Example 36 S-PESTEMPO-COOH 0.81 0.5 membrane Example 37 S-PES TEMPO 0.81 0.6 membraneExample 38 S-PES PROXYL-CONH₂ 0.85 0.5 membrane Example 39 S-PESPROXYL-COOH 0.86 0.5 membrane Example 40 S-PES ***) 0.95 0.7 membraneExample 41 S-PES DTBN 0.80 0.8 membrane Example 42 S-PES NHPI 1.34 6.5membrane Example 43 S-PES NHMI 1.34 5.2 membrane Example 44 S-PES NHSI1.36 5.4 membrane Comparative S-PES None — 2.5 example 5 membrane***) 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-yloxy

In example 35 to example 41, their concentrations of sulfate iondetected after 24 hours were within 0.5 to 0.7 ppm. To the contrary, inthe comparative example 5 in which no compound for decomposition ofactive oxygen was added, 2.5 ppm of sulfate ion was detected Comparisonbetween examples 35 to 41 and comparative example 6 has proven that eachcompound shown in examples 35 to 41 decomposed active oxygen, preventingoxidation of electrolyte membrane. Examples 42 to 44 are examples inwhich a compound having a redox potential of 1.0V or more was added.Compounds used in examples 42 to 44 have redox potentials of 1.34V,1.35V, and 1.35V, respectively. This redox potential is a potential foroxidizing S-PES membrane, and decomposes the electrolyte membrane.Therefore, in examples 42 to 44, greater sulfate ions were detected thancomparative example 5 containing no compound for decomposition of activeoxygen.

Next, for examples 45 to 54 using Nafion® membranes as electrolytemembranes, and comparative example 6, employed compounds fordecomposition of active oxygen, redox potentials of compounds, andconcentrations of fluorine ion detected after hydrogen peroxideendurance are listed in Table 7. TABLE 7 Concen- trations Compounds forRedox of Electrolyte decomposition of potentials of fluorine membranesactive oxygen compounds ion Example 45 Nafion TEMPO-OH 0.81 0.3 membraneExample 46 Nafion TEMPO-COOH 0.81 0.2 membrane Example 47 Nafion TEMPO0.81 0.3 membrane Example 48 Nafion PROXYL-CONH₂ 0.85 0.2 membraneExample 49 Nafion PROXYL-COOH 0.86 0.2 membrane Example 50 Nafion ***)0.95 0.4 membrane Example 51 Nafion DTBN 0.80 0.4 membrane Example 52Nafion NHPI 1.34 0.3 membrane Example 53 Nafion NHMI 1.34 0.4 membraneExample 54 Nafion NHSI 1.36 0.3 membrane Comparative Nafion None — 1.5example 6 membrane***) 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-yloxy

In example 45 to example 51, their concentrations of fluorine iondetected after 24 hours were within 0.2 to 0.4 ppm. To the contrary, inthe comparative example 6 in which no compound for decomposition ofactive oxygen was added, 1.5 ppm of fluorine ion was detected.Comparison between examples 45 to 51 and comparative example 6 hasproven that each compound shown in examples 45 to 51 decomposed activeoxygen, preventing oxidation of electrolyte membrane. Examples 52 to 54are examples in which a compound having a redox potential of 1.0V ormore was added. Concentrations of fluorine ion detected in examples 52to 54 were within 0.3 to 0.4 ppm, and substantially equivalent toexamples 45 to 51, thus showing an identical level ofoxidation-preventive effect to examples 45 to 51. It is suggested thatfluorine system electrolyte membranes are strong in resistance tooxidation relative to hydrocarbon system electrolyte membranes, and evenif the redox potential of a compound added for decomposition of activeoxygen is somewhat high, the electrolyte membrane will not be oxidizedby the compound.

Next, using membranes obtained from examples of embodiment andcomparative examples as electrolyte membranes, and platinum-supportingcarbon as electrodes, unit cells for fuel cell were prepared, andsubjected to a start and stop repeating endurance test. On electrolytemembrane obtained, platinum-supporting carbon (20 wt % Pt/Vulcan XC-72,Cabot Co.) was coated by a spread of 1 mg/cm² to the sides to be anodeand cathode, to fabricate a membrane electrode assembly (MEA).Fabricated MEA was assembled in a single cell, to provide a unit cellfor PEFC to be used for evaluation. The unit cell was a 5 cm² simplexcell.

The start and stop repeating endurance test was made in the followingmanner.

<Start and Stop Ring Endurance Test>

70° C. humidified hydrogen gas (atmospheric pressure) as an anode gasand 70° C. humidified oxygen gas (atmospheric pressure) as a cathode gaswere supplied to a unit cell held 70° C., and an open-circuit conditionwas held for 30 minutes, to start the test. In the test, supplying gasto the unit cell by a flow rate of 300 dm³/min, the current density wasincreased from a discharge open-circuit condition, making discharge tillthe terminal voltage gets below 0.3V. Then, after the terminal voltagehad got below 0.3V, an open-circuit condition was again held for 5minutes. This operation was re d, for comparison of the enduranceperformance in terms of the number of times when the voltage gets below0.4V under a condition of power generation with a current density of 1mA/cm³. It is noted that although S-PES membranes were used as theywere, each Nafion® membrane was used after exchange to Fe²⁺ type, forpromotion of the endurance test.

For examples 35 to 44 using S-PES membranes and comparative example 5,their compounds for decomposition of active oxygen and start-stoprepetition time numbers are listed in Table 8 below. TABLE 8 Compoundsfor Start-stop Electrolyte decomposition of repetition membranes activeoxygen time numbers Example 35 S-PES membrane TEMPO-OH 400 Example 36S-PES membrane TEMPO-COOH 420 Example 37 S-PES membrane TEMPO 410Example 38 S-PES membrane PROXYL-CONH₂ 410 Example 39 S-PES membranePROXYL-COOH 410 Example 40 S-PES membrane ***) 380 Example 41 S-PESmembrane DTBN 390 Example 42 S-PES membrane NHPI 30 Example 43 S-PESmembrane NHMI 30 Example 44 S-PES membrane NHSI 30 Comparative S-PESmembrane None 50 example 5***) 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-yloxy

For the comparative example 5 in which no compound for decomposition ofactive oxygen was added, under the condition of power generation with acurrent density of 1 mA/cm³, the voltage dropped below 0.4 V at astart-stop repetition time number of 50. To the contrary, in each ofexample 35 to example 41, the voltage dropped below 0.4V at a start-stoprepetition time number over 400, as a verification of effectiveness inthe power generation test as well. In each of example 42 to example 44in which a compound having a redox potential higher than 1.0V was added,the voltage dropped below 0.4V at a start-stop repetition time number of30, failing to obtain an effective result.

For examples 45 to 54 using Nafion® membranes and comparative example 6,their compounds for decomposition of active oxygen and start-stoprepetition time numbers are listed in Table 9 below. TABLE 9 Compoundsfor Start-stop Electrolyte decomposition of repetition membranes activeoxygen time numbers Example 45 Nafion membrane TEMPO-OH 550 Example 46Nafion membrane TEMPO-COOH 580 Example 47 Nafion membrane TEMPO 570Example 48 Nafion membrane PROXYL-CONH₂ 580 Example 49 Nafion membranePROXYL-COOH 580 Example 50 Nafion membrane ***) 550 Example 51 Nafionmembrane DTBN 530 Example 52 Nafion membrane NHPI 550 Example 53 Nafionmembrane NHMI 510 Example 54 Nafion membrane NHSI 580 Comparative Nafionmembrane None 70 example 6***) 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-yloxy

For the comparative example 6 in which no compound for decomposition ofactive oxygen was added, under the condition of power generation with acurrent density of 1 mA/cm³, the voltage dropped below 0.4 V at astart-stop repetition time number of 70. To the contrary, in each ofexample 45 to example 54 in which a compound for decomposition of activeoxygen was added, the voltage dropped below 0.4V at a start-stoprepetition time number over 500, as a verification of effectiveness inthe power generation test as well. Also in example 52 to example 54 inwhich a compound having a redox potential higher than 1.0V was added,the voltage dropped below 0.4V at a start-stop repetition time numberover 500, with a verification of effectiveness encompassing the case inwhich the compound for decomposition of active oxygen has a redoxpotential higher than 1.0V, as well, under the use of Nafion® membraneas an electrolyte membrane.

As will be seen from the foregoing description, by using a hydrocarbonsystem electrolyte membrane such as an S-PES film developed for fuelcells, having this contain a compound with a redox potential within arange of 0.68V to 1.00V, the deterioration by oxidation of electrolytemembrane can be suppressed, allowing for an enhanced durability of fuelcell. Moreover, it is seen that this compound can be reversibly redoxedto allow the effect to be maintained over a long term Further, it alsois seen that this compound can suppress deterioration by oxidation ofelectrolyte membrane even in use of a fluorine system electrolytemembrane, and not simply in the case of fluorine system electrolytemembrane, but also allows for a prevented oxidation of hydrocarbonsystem electrolyte membrane.

The whole contents of Japanese Patent Application No. 2004-203151(application date: Jul. 9, 2004), Japanese Patent Application No.2004-258507 (application date: Sep. 6, 2004), Japanese PatentApplication No. 2004-349842 (application date: Dec. 2, 2004), JapanesePatent Application No. 2005-053653 (application date: Feb. 28, 2005),and Japanese Patent Application No. 2005-172229 (application date: Jun.13, 2005) are incorporated herein by reference.

While the contents of the present invention have been described by wayof mode of embodiments and example thereof such description is notlimited thereto, and it will be obvious for artisan that various changesand improvements can be made.

INDUSTRIAL APPLICABILITY

In a fuel system according to the invention, an antioxidant residing inor contacting a solid polymer electrolyte membrane decomposes activeoxygen. Further, after the decomposition of active oxygen, theantioxidant returns to an original form by a redox cycle of theantioxidant itself, and can be used many times. This allows forimplementation of a fuel cell system with a maintained durability.

1. A fuel cell system characterized by a fuel cell having a solidpolymer electrolyte membrane, and an antioxidant residing in orcontacting the solid polymer electrolyte membrane, for inactivatingactive oxygen.
 2. The fuel cell system as claimed in claim 1,characterized by the fuel cell comprising a plurality of laminated unitcells each respectively comprising a membrane electrode assemblycomprising the solid polymer electrolyte membrane, and an air electrodeand a fuel electrode, with the solid polymer electrolyte membrane inbetween, an air electrode side separator disposed on the air electrodeside of the membrane electrode assembly, cooperating with the membraneelectrode assembly to have an air channel defined therebetween, and afuel electrode side separator disposed on a surface at the fuelelectrode side of the membrane electrode assembly, cooperating with themembrane electrode assembly to have a fuel gas channel definedtherebetween, and an antioxidant supply system configured to supply thefuel cell with the antioxidant, having the antioxidant contacting thesolid polymer electrolyte membrane.
 3. The fuel cell system as claimedin claim 1, characterized by the fuel cell comprising a plurality oflaminated unit cells each respectively comprising a membrane electrodeassembly comprising the solid polymer electrolyte membrane, and an airelectrode and a fuel electrode, with the solid polymer electrolytemembrane in between, an air electrode side separator disposed on the airelectrode side of the membrane electrode assembly, cooperating with themembrane electrode assembly to have an air channel defined therebetween,and a fuel electrode side separator disposed on a surface at the fuelelectrode side of the membrane electrode assembly, cooperating with themembrane electrode assembly to have a fuel gas channel definedtherebetween, and antioxidant supply means for supplying the fuel cellwith the antioxidant, having the antioxidant contacting the solidpolymer electrolyte membrane.
 4. The fuel cell system as claimed inclaim 2, characterized in that the antioxidant is supplied as anantioxidant solution continuously to the fuel electrode.
 5. The fuelcell system as claimed in claim 4, characterized in that the antioxidantsolution is an aqueous solution.
 6. The fuel cell system as claimed inclaim 4, characterized in that the antioxidant supply system comprisesan antioxidant solution tank having the antioxidant solution sealedtherein, a liquid feed pump for feeding the antioxidant solution to thefuel electrode, an antioxidant solution line interconnecting theantioxidant solution tank and the liquid feed pump, and an antioxidantsolution line interconnecting the liquid feed pump and the fuel gaschannel.
 7. The fuel cell system as claimed in claim 2, characterized inthat the antioxidant comprises a hydrocarbon system compound composed offour elements being carbon, oxygen, nitrogen, and hydrogen.
 8. The fuelcell system as claimed in claim 2, characterized in that an oxidant ofthe antioxidant is hydrolyzed to a chemically stable hydrolysate.
 9. Thefuel cell system as claimed in claim 2, characterized in that theantioxidant has a reversible redox-ability, and an oxidant of theantioxidant is chemically stable.
 10. The fuel cell system as claimed inclaim 9, characterized in that an oxidant of the antioxidant or unusedantioxidant is oxidized by a catalyst contained in the air electrode, tobe discharged as CO₂, H₂O, or N₂.
 11. The fuel cell system as claimed inclaim 2, characterized in that the antioxidant has a standardoxidation-reduction potential greater than 0.68 V (NHE) and smaller than1.77 V (NHE).
 12. The fuel cell system as claimed in claim 2,characterized in that the antioxidant comprises a compound representedby a general formula (I) below

where R₁ and R₂ denote arbitrary substituent groups, identical ordifferent, and X denotes an oxygen atom or a hydroxyl group.
 13. Thefuel cell system as claimed in claim 12, characterized in that R₁ and R₂are combined with each other, to form a double bond, an aromatic ring,or a nonaromatic ring.
 14. The fuel cell system as claimed in claim 13,characterized in that the antioxidant comprises an imide compoundrepresented by a general formula (II) below

where a ring Y₁ denotes any one kind of ring among 5-membered to12-membered rings double-bonded, aromatic or nonaromatic.
 15. The fuelcell system as claimed in claim 14, characterized in that theantioxidant comprises an imide compound represented by a general formula(III) below

where R₃ and R₄ respectively denote elements of a set of hydrogen atoms,halogen atoms, alkyl groups, aryl groups, cycloalkyl groups, hydroxylgroups, alkoxyl groups, carboxyl groups, alkoxycarbonyl groups, or acylgroups, mutually identical or different, X denotes an oxygen atom or ahydroxyl group, and n denotes an integer within 1 to
 3. 16. The fuelcell system as claimed in claim 15, characterized in that R₃ and R₄ arecombined with each other to form a double bond, an aromatic ring, or anonaromatic ring.
 17. The fuel cell system as claimed in claim 15,characterized in that R₃ and R₄ are combined with each other to form anyone kind of ring among 5-membered to 12-membered rings aromatic ornonaromatic.
 18. The fuel cell system as claimed in claim 15,characterized in that R₃ and R₄ are combined with each other to form atleast one kind of ring selective from a set of a cycloalkane, acycloalkene, a bridged hydrocarbon ring, and an aromatic ring, andsubstitutions thereof.
 19. The fuel cell system as claimed in claim 15,characterized in that the imide compound comprises an imide compoundrepresented by one of general formulas (IVa) to (IVf) below

where R₃ to R₆ respectively denote elements of a set of hydrogen atoms,halogen atoms, alkyl groups, hydroxyl groups, alkoxyl groups, carboxylgroups, alkoxycarbonyl groups, acyl groups, nitro groups, cyano groups,or amino groups, mutually identical or different, and n denotes aninteger within 1 to
 3. 20. The fuel cell system as claimed in claim 15,characterized in that the imide compound comprises an imide compoundselective from a set of N-hydroxy succinic acid imide, N-hydroxy maleicacid imide, N-hydroxy hexahydrophthalic acid imide,N,N′-dihydroxycyclohexane tetracarboxylic acid imide,N-hydroxyphthalimide, N-hydroxy tetrabromophthalic acid imide, N-hydroxytetrachlorophthalic acid imide, N-hydroxy fatty acid imide, N-hydroxyhimic acid imide, N-hydroxy trimellitic acid imide, N, N′-dihydroxypyromellitic acid imide, and N,N′-dihydroxynaphthalene tetracarboxylicacid imide.
 21. The fuel cell system as claimed in claim 14,characterized in that the compound represented by the general formula(II) comprises a compound represented by a general formula (V) below

where X denotes an oxygen atom or a hydroxyl group, R₁ to R₆respectively denote elements of a set of hydrogen atoms, alkyl groups,aryl groups, cycloalkyl groups, hydroxyl groups, alkoxyl groups,carboxyl groups, substituent carbonyl groups, acyl groups, or acyloxygroups, mutually identical or different, wherein at least two of R₁ toR₆ may be combined with each other to form a double bond, or an aromaticor nonaromatic ring, whereof at least one ring may comprises anN-substituent cyclic imide group.
 22. The fuel cell system as claimed inclaim 21, characterized in that the compound represented by the generalformula (V) comprises a compound represented by one of general formulas(VIa) and (VIb) below

where R₇ to R₁₂ respectively denote elements of a set of hydrogen atoms,alkyl groups, hydroxyl groups, alkoxyl groups, carboxyl groups,alkoxycarbonyl groups, acyl groups, nitro groups, cyano groups, or aminogroups, mutually identical or different.
 23. The fuel cell system asclaimed in claim 22, characterized in that the compound represented byone of the general formulas (V), (VIa) and (VIb) comprises an imidecompound selective from a set of N-hydroxyglutaric acid imide,N-hydroxy-1,8-naphthalene dicarboxylic acid imide, N-hydroxy-1,8-decalindicarboxylic acid imide, N, N′-dihydroxy-1,8,4,5-naphthalenetetracarboxylic acid imide, N,N′-dihydroxy-1,8,4,5-decalintetracarboxylic acid imide, and N,N′,N″-trihydroxy isocyanuric acidimide.
 24. The fuel cell system as claimed in claim 2, characterized inthat the antioxidant has an oxidation-reduction potential greater than0.68 V (NHE) and smaller than 1.00 V (NHE).
 25. The fuel cell system asclaimed in claim 12, characterized in that the compound represented bythe general formula (I) comprises a compound represented by a generalformula (VII) below

where R₁₃ and R₁₄ each respectively denote an alkyl group, or an alkylgroup substituted in part by an arbitrary radical, wherein R₁₃ and R₁₄may be chained, ringed, or branched, wherein R₁₃ and R₁₄ may be combinedwith each other to form a ring, wherein oxygen and nitrogen atoms may beincluded.
 26. The fuel cell system as claimed in claim 25, characterizedin that the compound represented by the general formula (VII) comprisesa compound represented by a general formula (VIII) below

where R₁₃ to R₁₆ each respectively denote an alkyl group, or an alkylgroup substituted in part by an arbitrary radical, wherein R₁₃ to R₁₆may be chained, ringed, or branched, wherein R₁₃ and R₁₄, or R₁₅ and R₁₆may be combined with each other to form a ring, wherein oxygen andnitrogen atoms may be included.
 27. The fuel cell system as claimed inclaim 26, characterized in that the compound represented by the generalformula (VIII) comprises a compound represented by a general formula(IX) below

where a ring Y₂ denotes a 5-membered or 6-membered ring formed by R₁₃and R₁₄ mutually combined.
 28. The fuel cell system as claimed in claim27, characterized in that the compound represented by the generalformula (IX) comprises a compound represented by a general formula (X)below

where Z denotes a substituent group selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, amino groups, andsubstituent groups including a hydrogen atom, wherein for Z being analkyl group, the alkyl group may be substituted in part by an arbitraryradical, and may be chained, ringed, or branched in part, and mayinclude oxygen and nitrogen atoms, wherein for Z being an aryl group,the aryl group may be substituted in part by an arbitrary radical, andmay include oxygen and nitrogen atoms.
 29. The fuel cell system asclaimed in claim 27, characterized in that the compound represented bythe general formula (IX) comprises a compound represented by a generalformula (XI) below

where Z denotes a substituent group selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, amino groups, andsubstituent groups including a hydrogen atom, wherein for Z being analkyl group, the alkyl group may be substituted in part by an arbitraryradical, and may be chained, ringed, or branched in part, and mayinclude oxygen and nitrogen atoms, wherein for Z being an aryl group,the aryl group may be substituted in part by an arbitrary radical, andmay include oxygen and nitrogen atoms.
 30. The fuel cell system asclaimed in claim 27, characterized in that the compound represented bythe general formula (IX) comprises a compound represented by a generalformula (XII) below

where Z denotes a substituent group selective from a set of alkylgroups, aryl groups, alkoxy groups, carboxyl groups, alkoxycarbonylgroups, cyano groups, hydroxyl groups, nitro groups, amino groups, andsubstituent groups including a hydrogen atom, wherein for Z being analkyl group, the alkyl group may be substituted in part by an arbitraryradical, and may be chained, ringed, or branched in part, and mayinclude oxygen and nitrogen atoms, wherein for Z being an aryl group,the aryl group may be substituted in part by an arbitrary radical, andmay include oxygen and nitrogen atoms.
 31. The fuel cell system asclaimed in claim 2, characterized in that the fuel cell comprises anyone kind selective from among a hydrogen type, a direct methanol type,and a direct hydrocarbon type.
 32. The fuel cell system as claimed inclaim 1, characterized in that the antioxidant comprises a compoundhaving a redox cycle, where it acts as a reducing agent in a range ofpotentials lower than a redox potential of hydroxy radical, and as anoxidizing agent in a range of potentials higher than a redox potentialwhere hydrogen peroxide acts as a reducing agent, and the solid polymerelectrolyte contains said compound as an oxidation-reduction catalyst.33. A solid polymer electrolyte characterized by a compound having aredox cycle, where it acts as a reducing agent in a range of potentialslower than a redox potential of hydroxy radical, and as an oxidizingagent in a range of potentials higher than a redox potential wherehydrogen peroxide acts as a reducing agent.
 34. The solid polymerelectrolyte as claimed in claim 33, characterized in that said compoundhas a redox potential greater than 0.68V (NHE), and smaller than 2.85V(NHE).
 35. The solid polymer electrolyte as claimed in claim 34,characterized in that said compound acts as a reducing agent in a rangeof potentials lower than a redox potential where hydrogen peroxide actsas an oxidizing agent, and as an oxidizing agent in a range ofpotentials higher than a redox potential where hydrogen peroxide acts asa reducing agent.
 36. The solid polymer electrolyte as claimed in claim35, characterized in that said compound has a redox potential greaterthan 0.68V (NHE), and smaller than 1.77V (NHE).
 37. The solid polymerelectrolyte as claimed in claim 36, characterized in that said compoundhas a redox potential greater than 0.68V (NHE), and smaller than 1.00V(NHE).
 38. The solid polymer electrolyte as claimed in claim 33,characterized in that said compound comprises a compound represented bya general formula (I) below

where R₁ and R₂ denote arbitrary substituent groups, identical ordifferent, and X denotes an oxygen atom or a hydroxyl group.
 39. Thesolid polymer electrolyte as claimed in claim 36, characterized in thatR₁ and R₂ are combined with each other, to form a double bond, anaromatic ring, or a nonaromatic ring.
 40. The solid polymer electrolyteas claimed in claim 38, characterized in that said compound comprises animide compound represented by a general formula (II) below

where a ring Y₁ denotes any one kind of ring among 5-membered to12-membered rings double-bonded or aromatic or nonaromatic.
 41. Thesolid polymer electrolyte as claimed in claim 40, characterized in thatthe compound represented by the general formula (II) comprises an imidecompound represented by a general formula (III) below

where R₃ and R₄ respectively denote elements of a set of hydrogen atoms,halogen atoms, alkyl groups, aryl groups, cycloalkyl groups, hydroxylgroups, alkoxyl groups, carboxyl groups, alkoxycarbonyl groups, or acylgroups, mutually identical or different, X denotes an oxygen atom or ahydroxyl group, and n denotes an integer within 1 to
 3. 42. The solidpolymer electrolyte as claimed in claim 41, characterized in that R₃ andR₄ are combined with each other to form a double bond, an aromatic ring,or a nonaromatic ring.
 43. The solid polymer electrolyte as claimed inclaim 41, characterized in that R₃ and R₄ are combined with each otherto form any one kind of ring among 5-membered to 12-membered ringsaromatic or nonaromatic.
 44. The solid polymer electrolyte as claimed inclaim 41, characterized in that R₃ and R₄ are combined with each otherto form at least one kind of ring selective from a set of a cycloalkane,a cycloalkene, a bridged hydrocarbon ring, and an aromatic ring, andsubstitutions thereof.
 45. The solid polymer electrolyte as claimed inclaim 41, characterized in that the imide compound comprises an imidecompound represented by one of general formulas (IVa) to (IVf) below

where R₃ to R₆ respectively denote elements of a set of hydrogen atoms,halogen atoms, alkyl groups, hydroxyl groups, alkoxyl groups, carboxylgroups, alkoxycarbonyl groups, acyl groups, nitro groups, cyano groups,or amino groups, mutually identical or different, and n denotes aninteger within 1 to
 3. 46. The solid polymer electrolyte as claimed inclaim 41, characterized in that the imide compound comprises an imidecompound selective from a set of N-hydroxy succinic acid imide,N-hydroxy maleic acid imide, N-hydroxy hexahydrophthal imide,N,N′-dihydroxycyclohexane tetracarboxylic acid imide,N-hydroxyphthalimide, N-hydroxy tetrabromophthalic acid imide, N-hydroxytetrachlorophthalic acid imide, N-hydroxy fatty acid imide, N-hydroxyhimic acid imide, N-hydroxy trimellitic acid imide, N,N′-dihydroxypyromellitic acid imide, and N,N′-dihydroxynaphthalene tetracarboxylicacid imide.
 47. The solid polymer electrolyte as claimed in claim 40,characterized in that the compound represented by the general formula(II) comprises a compound represented by a general formula (V) below

where X denotes an oxygen atom or a hydroxyl group, R₁ to R₆respectively denote elements of a set of hydrogen atoms, alkyl groups,aryl groups, cycloalkyl groups, hydroxyl groups, alkoxyl groups,carboxyl groups, substituent carbonyl groups, acyl groups, or acyloxygroups, mutually identical or different, wherein at least two of R₁ toR₆ may be combined with each other to form a double bond, or an aromaticor nonaromatic ring, whereof at least one ring may comprises anN-substituent cyclic imide group.
 48. The solid polymer electrolyte asclaimed in claim 47, characterized in that the compound represented bythe general formula (V) comprises a compound represented by one ofgeneral formulas (VIa) and (VIb) below

where R₇ to R₁₂ respectively denote elements of a set of hydrogen atoms,alkyl groups, hydroxyl groups, alkoxyl groups, carboxyl groups,alkoxycarbonyl groups, acyl groups, nitro groups, cyano groups, or aminogroups, mutually identical or different.
 49. The solid polymerelectrolyte as claimed in claim 48, characterized in that the compoundrepresented by one of the general formulas (V), (VIa) and (VIb)comprises an imide compound selective from a set of N-hydroxyglutaricacid imide, N-hydroxy-1,8-naphthalene dicarboxylic acid imide,N-hydroxy-1,8-decalin dicarboxylic acid imide,N,N′-dihydroxy-1,8,4,5-naphthalene tetracarboxylic acid imide,N,N′-dihydroxy-1,8,4,5-decalin tetracarboxylic acid imide, andN,N′,N″-trihydroxy isocyanuric acid imide.
 50. The solid polymerelectrolyte as claimed in claim 38, characterized in that the compoundrepresented by the general formula (I) comprises a compound representedby a general formula (VII) below

where R₁₃ and R₁₄ each respectively denote an alkyl group, or an alkylgroup substituted in part by an arbitrary radical, wherein R₁₃ and R₁₄may be chained, ringed, or branched, wherein R₁₃ and R₁₄ may be combinedwith each other to form a ring, wherein oxygen and nitrogen atoms may beincluded.
 51. The solid polymer electrolyte as claimed in claim 50,characterized in that the compound represented by the general formula(VII) comprises a compound represented by a general formula (VIII) below

where R₁₃ to R₁₆ each respectively denote an alkyl group, or an alkylgroup substituted in part by an arbitrary radical, wherein R₁₃ to R₁₆may be chained, ringed, or branched, wherein R₁₃ and R₁₄, or R₁₅ and R₁₆may be combined with each other to form a ring, wherein oxygen andnitrogen atoms may be included.
 52. The solid polymer electrolyte asclaimed in claim 51, characterized in that the compound represented bythe general formula (VIII) comprises a compound represented by a generalformula (IX) below

where a ring Y₂ denotes a 5-membered or 6-membered ring formed by R₁₃and R₁₄ mutually combined.
 53. The solid polymer electrolyte as claimedin claim 52, characterized in that the compound represented by thegeneral formula (IX) comprises a compound represented by a generalformula (X) below

where Z denotes a kind of substituent group selective from a set ofalkyl groups, aryl groups, alkoxy groups, carboxyl groups,alkoxycarbonyl groups, cyano groups, hydroxyl groups, nitro groups,amino groups, and substituent groups including a hydrogen atom, whereinfor Z being an alkyl group, the alkyl group may be substituted in partby an arbitrary radical, and may be chained, ringed, or branched inpart, and may include oxygen and nitrogen atoms, wherein for Z being anaryl group, the aryl group may be substituted in part by an arbitraryradical, and may include oxygen and nitrogen atoms.
 54. The solidpolymer electrolyte as claimed in claim 52, characterized in that thecompound represented by the general formula (IX) comprises a compoundrepresented by a general formula (XI) below

where Z denotes a kind of substituent group selective from a set ofalkyl groups, aryl groups, alkoxy groups, carboxyl groups,alkoxycarbonyl groups, cyano groups, hydroxyl groups, nitro groups,amino groups, and substituent groups including a hydrogen atom, whereinfor Z being an alkyl group, the alkyl group may be substituted in partby an arbitrary radical, and may be chained, ringed, or branched inpart, and may include oxygen and nitrogen atoms, wherein for Z being anaryl group, the aryl group may be substituted in part by an arbitraryradical, and may include oxygen and nitrogen atoms.
 55. The solidpolymer electrolyte as claimed in claim 52, characterized in that thecompound represented by the general formula (IX) comprises a compoundrepresented by a general formula (XII) below

where Z denotes a kind of substituent group selective from a set ofalkyl groups, aryl groups, alkoxy groups, carboxyl groups,alkoxycarbonyl groups, cyano groups, hydroxyl groups, nitro groups,amino groups, and substituent groups including a hydrogen atom, whereinfor Z being an alkyl group, the alkyl group may be substituted in partby an arbitrary radical, and may be chained, ringed, or branched inpart, and may include oxygen and nitrogen atoms, wherein for Z being anaryl group, the aryl group may be substituted in part by an arbitraryradical, and may include oxygen and nitrogen atoms.
 56. A fuel cellcomprising a solid polymer electrolyte claimed in claim
 33. 57. A fuelcell vehicle having mounted thereon a fuel cell system claimed in claim1.